64 bit is the right choice in 90% of situations these days. One or two years ago I would say go for 32 bit (64 bit server but run 32 bit software there) because there were still compatibility issues with many applications.
Based on the only fully comprehensive set of benchmarks from when 64bit first became mainstream, 32bit generally had better performance with the exception of memory intensive applications like databases. The reasons for the difference in performance were rooted in the architecture rather than the software, so I don't see any reason for this to be any less true today. It may be possible that a few applications that were borderline in respect to 32bit vs 64bit performance may now have a slight edge on 64bit, but I have yet to see anyone provide evidence of this one way or the other.
In short, if you need more than 4GB of memory for a single application, or run data intensive applications, go 64bit. Otherwise, stick with 32bit.
ASTUTE HOSTING: Advanced, customized, and scalable solutions with AS54527 Premium Canadian Optimized Network (Level3, PEER1, Shaw, Tinet) MicroServers.io: Enterprise Dedicated Hardware with IPMI at VPS-like Prices using AS63213 Affordable Bandwidth (Cogent, HE, Tinet) Dedicated Hosting, Colo, Bandwidth, and Fiber out of Vancouver, Seattle, LA, Toronto, NYC, and Miami