i'm going with DOA4 because i'm biased and plus i've never played Virtua Fighter 5 

I've never played Virtua Fighter 5, but I am quite the DOA fan. Guess what I picked.

I found DoA4 to be much more fun than Virtua Fighter 5. But that is me. 
 
The Quest mode in VF5 locks you into one character, and I didn't like that. I don't play either of these games online or competitively, I play around with friends, having a good time and picking different characters for kicks. The fact that DoA4 has the short arcade games, then some extra stuff was more for me. That and I really liked their lobby system online (when it wasn't laggy). I never even tried with the VF5 stuff. 
 
All that said, VF5 wasn't too bad, I just prefer DoA4.

VF5 is a really technical and masterful fighting game.  Very very few people can handle playing it and enjoy it.  Think Street Fighter 3 + Blazblue + learning real martial arts.  I can appreciate it at my low level of playing.  I bought a PS3 for it and imported the game if that's any indication of what I think of Virtua Fighter...
 
DOA4 is OK but it's so reliant on cheap stuff like the parry moves.  Fighting against the AI is annoying because winning just depends on if the AI parries you correctly or not.  Pretty similar for online play.
 
I vote VF5.

Play DOA4 if you want a game that's better at delivering eye candy than fighting.  Play VF5 if you want a super-technical, competitive experience and don't care about flash.

Why do you ask? Although I've played both series extensively, the AI in Dead or Alive 4 is quite unscrupulous; I'd consider that title a low point in the series.

Still, if you're asking which game is more difficult to master, it's always Virtua Fighter. Consider a technique such as the "evading throw escape guard." That's right. It is possible to evade an attack, buffer escapes for multiple throws, and simultaneously guard… all in what appears to be one fast movement. A casual observer would have no idea of this level of hidden complexity; in fact, a majority of people who play Virtua Fighter may never even realize such techniques exist, much less actually be able to execute them (with consistent regularity), but the option is always there.

A lot of people (i.e. Americans) make fun of Virtua Fighter, but the harsh truth of the matter is that they don't understand it. Amongst all the games I've ever played, the Virtua Fighter series is one of the most rewarding. I've been playing since the series' inception, though. You definitely have to put in the time…

Sorry to say this, but I vote for Virtua Fighter 5.

VF5 all the way. So deep, so rewarding and fun.

I have always enjoyed the Dead or Alive series most, but I really think Virtua Fighter is just a better fighter.  And Dead or Alive 3 was much better than 4 in my opinion.

DoA4 here. As someone who sucks at fighting games I enjoy the fact that the game is made to be accessible for people like me and have flashy, slick looking moves with limited experience. 
 
I also LOVE the training mode where it shows you the command, then lets you try it over and over and tells you when you get it right. I would be FAR more attracted to fighting games if more had something like this, but if I'm trying to pull off a move on a command list but I have no idea what it looks like, I often have no idea of whether or not I actually did it.

I think virtual fighter 5 is technically the better game in almost every way, however I had like way more fun with doa games. I like how defensive and counter attacking, also bewbs of doa games, it just has more style.

I S ranked VF5 and still occassionaly get my ass kicked online.   Guess which one I picked. 

VF5

VF5, Definetely.

Speaking of VF5, the new one came out in Japanese arcades like 3 weeks ago.  I wonder how it is.

Dead or Alive 4.

I'm giving to VF5, that series always just felt better to me.

VF5 hands down. DoA4 is good, but I find it to be more eye candy than anything. It also has a stupidly high barrier of just becoming decent. VF5 is easy to get into, hard to master, but fun the whole damn way through.

VF5 Beats DOA4 it's one better.  
 
Simple. 

VF5 is a legitimate fighter and DOA is more of a joke than Tekken. Pretty easy question to answer, unless you never played VF5 and still think anyone gives a damn about your opinion.