Free Net Poker

From both sites pokerstars is the better. I also like fulltilt, they are as good as stars and offer bigger bonus. You can find review of fulltilt here

Free Net Poker

Out of the two I would reccomend Pokerstars especially if your into tournament play as UB tends to freeze for a long time when you get moved to a new table. There are alot of other sites you can try also, go to one of these sites to check out reviews and other info.

Free Net Poker

If you like graphics and looks, pokerstars is way better than full tilt in this regard. Otherwise both meet your other criteria just fine. Like another has said, feel free to try both.

Edit: I just noticed you said you were thinking about playing UB. This is also fine for play money (not as good as stars though), but there has been evidence of cheating real-money players on that site, so I would hesitate playing for real money there.