FillTilt poker looks better. There are different view options that make the cards clear and easy to see the boards. It offers different variations of Poker, my fav being Hold'em.
It offers Ring games (cash games), SitnGo's, or Tourneys with up to 90 people in them. If you are looking for less Donkeys, you will have to build up your bankroll and play in the higher limits.
If you play with "play money" then you start playing the $250 games, and increase your bankroll. These have the weakest Donkeys. Then move to the $2,000 games which are much more stable. The $10,000 games have no donkeys as you only start with $1,000 play money, so they are good.
One problem though with Fulltilt is that occasionally the flops are odd. The boards are sometimes paired way too often (i.e. A-2-5-A-7), which means you can lose a lot on the river. Other people disagree with me about this, but maybe you will be lucky and not get these crazy flops.
Pokerstars looks like a 1980's Atari game, it is blocky, hard to see chips, cards, etc. It is hard to navigate the games, the options, the tourneys, etc.
Best thing about Fulltilt if you are from the UK is that everynight at 8.20pm they do Freerolls, where you enter a tourney for free (around 1k people) and win real $. However its tiny, like $2, but if you win enough, you build a little bankroll and play with real $$$!