Different strokes
Well, as another "Old SH3'er" who loves both, I can say that it depends on what you're looking for. At this point, SH5 is at the point where I can enjoy it a lot but, then again, it took quite a bit before SH3 reached that point too. It wasn't
always the nearly flawless wonder that it is now, and we can thank the modders for the fact that it ever got there. The same is the case with SH5. It has potential and, having gone through the process before, I'm quite confident that it will realize that potential eventually.
Right now, it's good enough for me to put down SH3 for, although it will be a very cold day in hell before I'll ever delete my first love, SH3, from my hard drive
What I truly love about SH5 is the immersion and the looks. For looks, this is not an actual photo, this is what it actually looks like on my currently rather modest rig. I'll start
really adding eye candy when my new GPU arrives:
This is only to illustrate my point about the looks. I'm very much in agreement with the sentiment that "looks aren't everything". If that hadn't been the case, I would have been totally infatuated with SH5 for much longer. And it's not like SH3 doesn't look good. It
still looks amazingly good to this very day, which in itself is very impressive.
On immersion, I really like the ability to be able to walk around my sub in a full 3D environment with fully functional (thanks to modders) dials, animated crew going about their business and all manner of other ambient stuff. But, even so, I'll be the first to admit that I find myself using the (again, thanks modders) teleport function when things get hectic or I just want to get on with things. The "dialogs with crew members?" They need work still, but it's a nice touch and, again, that can be modded too.
But, again, I'm a sucker for immersion. I spent hours memorizing the names of my SH3 crew, thinking up background stories for them, keeping track of their promotions etc., I guess I'm just into that sort of thing.
Now... Is it worth investing in at this point? At $5, what do you stand to lose?
The main drawbacks, to me, are that you have to be willing to invest a bit of time, OK more than just a bit, to get the modding done right at this time. But if you're a "veteran" of the SH3 days, that's no stranger to you. It's back to the "restart your campaigns over and over because somebody just came up with something that you just HAVE to have

" phase.
Or not, but you
will need some time to get at least the basics installed.
Also, just as was the case when SH3 came out, SH5 isn't exactly friendly to low to mid-spec machines. It's a resource hog. But, as I mentioned, when SH3 came out I considered myself blessed to get 17 FPS leaving Kiel. Now I can churn out 35 with my modest Phenom II X4 and GTX260, but that's with the graphics turned down quite a bit. A machine that runs everything
else quite well indeed. Still, referring back to the pic above, it's quite pretty with "low" graphics.
The lack of the TypII, IX and XXI? I miss the cute little Einbaum, but I never loved the IX all that much. Handled like a cow to me compared to the VII, but I'll be the first to agree that it should have been included at the very least. Hey, I liked it in SH3 just fine when I was off to Paukenschlag since I didn't have to rely on Milk Cows to get back again. Whether you personally want it or not, it's a matter of "don't limit the player's options" to me. It
should be there. I'm sure the modders will find a way to add it.
As to the campaign in SH5 ending in '43? That
was the case. There's a mod for that complete with missions and everything already. Personally, I never did make it past '43 in SH3 but, then again, I liked the fact that I had something to strive for. Arbitrarily ending your career "just because" is another case of "don't limit the player's options" and I'm glad it has been taken care of, even though I'm sure I
still won't ever make it to '44.
Bottom line? They're both good, depending on what you want. I'm not going to say that either is "better", but I would definitely say that SH5 is worth giving a spin at this point.
For what it's worth. YMMV
EDIT: And I should add that I agree wholeheartedly with Sailor Steve that SH4 has been sadly neglected. I don't know why, as it was clearly a leap forward from SH3. One thing that particularly appealed to me in SH4 compared to her predecessor was the concept of actual missions, as opposed to the old "patrol grid XXxx, get bored finding nothing, then go off to patrol your favorite hunting grounds, it's not like we care anyway". No, it's not like I hated SH3 because of that, it's just that I felt that my patrol orders were nothing more than "go there first, then do what you were planning to do anyway." Yes, I know, in sim terms "go to this location, press this button, then request new orders" isn't all that different, it's just that having an actual mission that you can actually fail (it's hard to fail plotting a navigation point on a map) works better for me. I wish that SH4 had caught on fire like SH3 and SH5 have.