The main difference is that the VIIC can upgrade to a much bigger conning tower/bridge than the VIIB, resulting in being able to carry the greatter AA firepower. I think everything else the upgrades are the same between the two, but I'm not 100% on that.
Don't forget to check the tech tree at the U-Boat upgrade screen, it might answer these questions for you.

Read both threads, some very sweet tonnage/renown in those patrols but unfortunate conclusion to the first one with the boat being destroyed by aircraft after a highly successful task force engagement.

I've been lucky enough, so far, to not take a direct hit from bombing though I have had crew killed by the shrapnel/concussion from near misses/nicks. I should qualify that: I've never had my boat destroyed directly by aircraft but they did play an indirect role in one patrol at scapa flow in an older campaign; after being shelled and damaged by a DD during a running gunfight I was rammed after firing two torps that missed and crash diving to late. My conning tower took the brunt of it and was nicely smashed up but there was also flooding in other compartments, possibly from the shelling, and my hull integretity was reduced to 16%. The destroyer wasn't so "lucky", and the beating he'd taken from our own deck gun had weakened his hull enough that he broke apart upon the collision (some small consolation).

With the DD out of action and the flooding I immediately blew balast and we managed to surface in one piece with most of the ship pumped out. Our AA guns were destroyed though and between that and the condition of the boat we were sitting ducks when one of the watch spotted an aircraft. I reflexively crash dived, having no way to fight back and in no condition to take another blow, and at 30 meters the lights blew out, the ship started coming apart and there were repeated calls of "man down". Blew ballast to no avail and the ship broke up at 40 meters depth.

My tendency to somewhat recklessly take on DDs has led to some extremely exciting surface battles but also to the premature end of more than a few careers. In the future I think I'll try and limit this to scenarios with lower risk, high reward and save the majority of the gunfights for armed merchants who have much less firepower and capability to ram :)

The unfortunate thing about the tech tree is that, while it does show what's available and roughly when, aside from new conning towers it doesn't link any of the tech to the boats. The wiki also states, in its tech development list, that certain tech may not be available on all boats so there really doesn't seem to be any definitive answers out there.

My gut instinct, based on my experience upto Nov '41 (the longest campaign I've run without getting KIA or "retiring") and the responses so far is that as the newer boats are developed the new tech will drop off and cease to be available on older models such as the VIIb. Otherwise the VIIc means quite a period of inactivity, while its being commissioned, just for improved AA and a 3 second faster dive time (but slightly slower top speeds). This was why I hadn't considered it worthwhile before, but in light of my thinking now that it may be able to use new tech that the VIIb can't, in addition to the improved AA, its worth the time in dry dock to wait for the new boat's comissioning and in the campaign I just started I'll go for it as soon as it becomes available (renown permitting).

Well, by the time I got around to switching to the VIIC, I'd already had two engine, two battery, one flak gun, radar, radar detector, hydrophone and one anechoic coating upgrades if I remember correctly. Everything I had on the VIIB I was able to repurchase on the VIIC. It cost me about 6000 renown on top of the 3000 to buy the boat, but I did it. I was also able to get the VIIC/3 conning tower, with two light flak emplacements on the bridge and the heavy flak on the wintergarten.

VIIB vs. VIIC

for me, the main thing is top speed - even more important than a 3 second dive time savings. With MAN upgrade the top speed for VIIB is 21kts in calm sea, and around 17 or 18 for VIIC in calm sea. VIIC is slower, which is really annoying when it comes to that sudden task-force detection and rush to intercept. 21kts also allows you to run away from smaller escorts on the surface. From what I've read on this thread, alberich and radar detector are available for VIIB. These are vital. The big question is: can VIIB use the schnorkel? Survival is next to impossible in the late war without the schnorkel, and hence if I can't have it on my VIIB, I will have to switch to a VIIC.

Second - I have the VIIB, and I never have the option to upgrade to VIIC. IXC is available, and I'm pretty sure that VIIC would be available if I had another boat (Mar 41). I think careers that start at this time get the VIIC. Does having the VIIB prevent the upgrade to VIIC? Will I have to buy an IXB and then a VIIC?

Third - VIIB looks better than the VIIC - the net-cutter looks wicked.

KB

P.S - does anyone ever use active radar or active sonar? I never do, but then again I don't do manual targetting.

Some more really good responses here and its nice to have it confirmed that the VIIb allows at least most of the upgrades (that we're aware of at this point in the thread) of the VIIc with the schnorkel being the question mark.

I like as much top as I can get too, although for a slightly different reason: going to flank speed on the surface, once detected, in my initial surface torpedo attack against convoys or unescorted merchants. There an extra few knots can mean the difference etween a close range beam shot hitting or missing and lessens exposure to gunfire from armed merchants and escorts.

At the same time I can see the merit of the improved flak in the VIIc, so perhaps the best time to upgrade isn't as soon as the VIIc becomes available but instead later in the war after the US enters and airborne ASW becomes a bigger threat (also if I'm not mistaken the VIIb/2 conning tower, with twin 20 cm flak stations is available before the VIIc/2).

Bumping my own thread just in case anyone has anything more to add or can answer some of the unanswered questions that have been raised.

This must be due to either your current flotilla (not all boats are used by all flotillas) or else to some mod installed that affects boat availability, or lastly a bug if you aren't using v 1.4b. In my campaign where I started in the 7th flotilla, in a VIIb, the VIIc was an available upgrade from it after I'd completely only a few patrols (so somewhere between Dec '39 - March '40 I think but long before Mar '41 in any case). I didn't upgrade to it because looking at its specs I didn't see the point, and instead later transferred to the 2nd Flotilla in Dec '40 and upgraded to a IXb and later IXc. Not sure of its precise availability or what other flotillas you can upgrade to it in (but definitely in the 7th you can go VIIb -> VIIc... maybe someone else can fill these blanks in).

YOu're right about that. Other flotillas (e.g. 29th) allow VIIB->VIIC.

Now - about the schorknel - I'm about to start a new boote - could someone tell me if VIIB can use Schnorkel?

Kb