I am VERY interested in both games. Although I'm a Mortal Kombat guy so I'll probably wait till April. I think if I had to choose between your 2 though, I would get UMVC3. I love the way the colors on this game stick out.
I was having the same situation a while back with deciding on which fighter to choose. I eventually settled on UMvC3. I did this for a number of reasons.
1. EVO: BlazBlue has been taken out of the EVO line up but UMvC3 hasn't so that does hinder the decision. If you're big on competitive gaming, you should know why EVO is so much of a problem.
2. I like how fast paced and how complicated and rewarding UMvC3 is. BlazBlue is a bit slower and has some really simple combos so it doesn't make it that much rewarding. Though you can argue that UMvC3 has that stupid "tap the screen to combo" thing, but that, in a sense, really hinders you from doing almost anything involving more than 5 hits and well it's just for noobs.
3. I just like the art style. It looks so crisp and highly detailed. BlazBlue is unique in its own way and looks just like the console versions, just like UMvC, but I've seen it all before with Guilty Gear (which I prefer over BB and Guilty Gear Accent Core, i think, will be on the PS Store as a PSP game so I'm just going to end up getting that).
4. I have a better time with UMvC3 as it feels more competitive and of course more hyped. Winning and making a name for yourself on UMvC is better than doing so on BB.
5. I'm certain a greater number of buyers will get UMvC3 so playing online will give you a player with a different fighting style than anyone else, giving it variety and complexity. Like, two people can have the same exact team of Dante, Virgil, and Deadpool, but play completely differently. Using combos and chains that you've never seen before as opposed to BB where you, for example, always get a Ragna who loves to spam Gauntlet Hades 30 times in a round.
And 6. With so many more characters in UMvC3 over BB and with a seemingly limitless number of teams that you can make, it just makes the game have that much more of an edge over BB. You won't see the same team used two or three times among different users in one day of playing online. This makes it really difficult and gives a greater rush keeping in mind what a specific character they have is weak against and strong with. Keeping you on your toes and making you really just have to pay attention and keep one step ahead of the competition.
And as an added bonus, if you like playing on the PSVita so much, you can connect that to the PS3 and use that as the controller on PS3's version of UMvC3. Kinda cool. And if you have BlazBlue Calamity Trigger for PS3, check out what I found here:http://community.us.playstation.com/message/37421821#37421821.
Of course, these are all just my opinions but anyway, I say get UMvC3. It's worth your while.
And add me on PSN, Xx_shawna_xX, if you ever do decide to get it and want a friend to play with. My team right now is X-23, Storm, and Dante.
I'd personally go for Ultimate Marvel vs. Capcom 3 for now. Mortal Kombat will be great too once it comes out.