Loved Tekken 5 Dark Resurrection but hated everything in Tekken 6 except the gameplay. Tekken Tag 2 isn't a main entry in the series so stuff like story mode is out the window and my expectations are different as a result. As long as Tekken Tag 2 plays great, has tons of characters and features a solid online mode I'll be happy.
Depends on what you mean by "story mode." Tekken Tag Tournament 2 is going to have the most CG endings out of any Tekken game yet. All 50+ characters have their own ending CGs, which together make up about 80 minutes of footage, according to Harada. Supposedly, watching them all together makes a story of sorts...but I can't say much about that, but it was said in an interview.
Anyway, I'd personally go for Tekken Tag Tournament 2 if I had to pick just one. Heck, I've already had mine pre-ordered for awhile. I do love Dead or Alive as well, though, so I understand how hard it can be to pick. Tekken Tag Tournament 2 definitely seems to have the most bang for your buck, though, and will more than likely have the larger user base. I see people on Tekken 6 and even Tekken 5: Dark Resurrection all of the time. There's still people on Dead or Alive 4 online, but from what I've seen, it just can't compare to how many play Tekken 6. Don't know how big a deal online is to you, though, MrCookiepants.
Both should be great experiences, though, if you love both series'. I've played the Alpha demo of Dead or Alive 5 quite a bit, and it's pretty nice. Doesn't feel
too different from the previous games, though. It's a beatiful game, too...though the color palette bugs me a bit. Everything seems too dark and gritty in comparison to the earlier games and stuff like Tekken Tag Tournament 2. Still, though, everything else looks pretty fantastic.
Which series have you enjoyed most in the past, though? If you ever enjoyed one more than the other, then go with that.
But if you can...get both. I'm definitely going to try and get Dead or Alive 5 as soon as possible after Tekken Tag Tournament 2.