I'd say go GK if you have limited funds, since you can buy fewer models worth more points. They're pretty boss right now and will probably hold up well over the long run. Personally I think 'Nids are decent right now but will probably do really well in 6th edition when it drops. Plus I'm an avid Tyranid player and love our 'underdog' army...so yea, Tyranids...good choice!