Both are GREAT, Bioshock has RPG and horror elements, and Dead Space, my favourite game this year is kind a DOOM sequel, but a LOT LOT FUN, HORROR, well developed for X360 and PS3.
Dead Space has no ONLINE gamming, but only the game itself RULES, you will play again at least 2 times.
GO GO GOOOOO
PSN: franco-br
MGS4, GH, MW2, GT5p, WipeoutHD, etc..etc..