OK, so I decided to go with NYGM Tonnage, for the simple reason that it seemed to offer much the same as GW in terms of gameplay changes, but with less of the slowness associated with improved hires graphics.
Love the damage mod. On my first patrol I torpedoed a lone tramp steamer that absolutely refused to sink. I ended up using 4 torpedoes (2 of which missed due to evasive action, thanks to my overeager deck gun crew alerting them), plus tons of 88mm shells. Damn thing just refused to go down. Thank God I was closer to Norway than Britain. Incidentally, I ended up having to fudge the logs a little bit - the attack started at about 0002 3 Sep 39 - well before I got Rules of Engagement orders from BdU. I'm also pretty sure that opening fire unannounced on a very tenuously ID'd merchant vessel from 2000+ meters in the middle of the nght was contrary to the Preisenordnung...
BdU still said I did a good job, though.