There are so many factors to consider when choosing a poker site, so I will try and make this as simple as possible.
1) Traffic - Nothing is worse than making a deposit on a site with no traffic. The two busiest online poker sites are Full Tilt Poker and Pokerstars. Pokerstars is #1 and has almost double the traffic of Full Tilt. On average there are approximately 150,000 people online at Stars. This means you don't ever have to wait for a game and the tournaments are the biggest (prizepool and player field) in online poker.
2) Games you will be playing. If you want to play anything other than Texas Holdem, Pokerstars and Full Tilt are your best bet. These sites have the most traffic at non-holdem games. If you want to play Sit N Go's - they fill fastest on Pokerstars.. which means you don't have to wait around forever.
3) Competition - If you are looking for sites with easier play, I personally say to try and avoid ones that do rakeback. I can't speak for Party Poker, but I know Pacific Poker has very soft competition and even Pokerstars if you pick your table properly.
4) Rakeback - Every hand you play on a poker site you contribute to something called a rake (the way poker sites make money). Rakeback simply means that you get some of this money back. Not every site offers rakeback, but if you want some sites that do, here is my
rakeback deals page.
5) Rewards / Loyalty Programs - Most of the sites that do not offer rakeback have amazing loyalty programs. For example, at Pokerstars you can actually get stuff in their store with minimal play. On Full Tilt, you need to put in like 500 hours to get a hat.
6) Software - Again, a personal thing... my favorites in this field are Stars, FTP and sites on the Merge Network.
I am sure I am missing a ton more things, but this should at least get you started. I recommend trying a few sites and seeing which YOU prefer. Finding a site that you are comfortable with can take time.
Thoughts on Party Poker / Poker Room:
Party Poker - Haven't played there in years so can't say too much. They took a big hit in traffic after they pulled out of the US, but recently they have changed the software and are doing some cool things to make it better. I still think this is one of the better sites, though they aren't what they were back in the day and Pokerstars has taken most of their market share.
Poker Room - One of the first sites I played on. If you can put up with the Ongame software (the software of the network that the site runs on - which I really don't like), there are some good overlay tournaments (smaller field with money added to the prize pool). Other than that, if this is the first site you are gonna play on, I would say check out stars or Full Tilt.
Good luck!