Eric,
I had a '94, 290 DA for about 3 years and have owned a '99 270 DA for 7 years. Both boats have been the single engine models, so I can't help you with questions about twins on these boats.
Here are some of my thoughts about your questions:
Cabin layout?
Pretty similar between the 2 boats. Personally, I prefer the 270 DA, for some reason it seems more open to me, however I had the dark blue interior and carpet on the 290 and I have the lighter colors on the 270, so the colors may have something to do with it.
Cockpit layout?
I prefer the 270 cockpit a lot more than the 290. I like the starboard side lounge, it provides great seating for guests who can talk with the captain, or the others in the seats behind the captain. The flip up captain seat easily holds 2 people, assuming they are not "super-sized" and enjoy each others company.
Midcabin functionality?
I prefer the 270. It is easier to access and seems just as roomy as the 290 was.
Sleeping space of forward berth?
The 290 has the edge here. The 270 V-berth is smaller. If you are 6 ft. you are going to have trouble sleeping in the 270 V-berth. Make sure you lay in the V-berth, preferably with your significant other and try it out before you buy if you are planning on sleeping up there. We always use the mid-cabin and have the kids sleep in the V-berth.
Performance with twin V6's?
Can't help you there, I was happy with the single on either of these boats and never felt the needs for twins. The 270 with it's counter-rotating props is far superior to the 290 as far as backing up or docking with a single.
Generator OK with twins? (space/weight)
Didn't have one on either boat.
Swim platform?
The 270 seems bigger and has worked out great.
As you can tell from my responses, I like my 270 more than my 290, which is probaby why I have had it longer and still have no desire to sell it. Inbetween the 290 and 270, I owned a '97 330 with V-drives. That is a great boat, but more than we needed for just cruising to resturants and going on day cruises.
Good luck, you won't be disappointed with either boat.