Keep in mind that if you buy the game from Steam, you will also need to buy all expansions from Steam: they're not compatible with the "regular" PC version.

Also, you can play all jobs on one character, so just try both (and all others that seem interesting to you) and see which you enjoy the most. But as said, you can't unlock SAM until you have at least one other job at 50, so might as well start with Monk (well Pugilist, and unlock Monk at 30).