I dont have any recent experience with
partypoker, so I cannot offer a direct comparison. Personally however I would start at PokerStars, if I were to do it all over again. PokerStars specifically have two game types, which are great for beginners, and that is the 2NL-5NL full ring cash games and 45 man SnGs.
2NL-5NL cash games on Stars are great, because the rake is low. 3,5% for 2NL and 4,15% for 5NL. PartyPoker charge the more industry typical 5% across the board. This matter, because a 5% rake cut up to 10BB/100 out of your winrate, so it makes it more difficult to beat the games and start building a bankroll.
I also recommend starting with full ring cash games, because its somewhat easier and more straight forward than playing 6-max. On PartyPoker than is not even an option, which to me is definitely an argument for choosing PokerStars as a beginner.
45 man SnGs on Stars are great, because they run all the time, and the rake is reasonable like 9c for a 1$ SnG. Single table SnGs on Stars are however not great for beginners, because then they charge 15c for a 1$ SnG and 21c for a 1,5$ SnG. So if single table SnGs is your preferred game, you might want to start somewhere else.
You might also want to consider moving somewhere else, when you have played for a while and build your bankroll up to something like 300-500$. If you play 10NL cash games, now the rake on PokerStars is 4,5%, and then it might matter more, that other sites such as PartyPoker offer better reward programs.
You might also want to start playing tournaments, and in my opinion PokerStars is not a good site for that. The rake for small stakes tournaments on PokerStars is 20% higher than anywhere else, and the fields are also very large, which increase variance as well as the amount of time, you need to dedicate to the task.
I understand, I have talked a lot about rake, and it might seem very nit-picky to complain about differences like paying 15c instead of 10c for a 1$ SnG. But really we need to see ourselfes as customers, and you would probably not choose to pay 50% more for your groceries just to shop in a different store. So this is actually a main reason for choosing one
poker site over another, especially since they essentially all offer the same sort of games.
