Google claims the 64-bit version is faster, more stable, more secure, and has better memory management.
It'll be more secure and have better memory management, sure. But faster? Hell, even 32-bit browsers scream on ahead and load up information in less than a split second. While it might officially be faster according to benchmarks, it won't make that huge of a difference in practice.
The biggest advantage won't be speed, but workload. 64-bit browsers will be able to handle a lot more open tabs running data heavy webpages with more grace than a 32-bit browser would.