I know it is said that a 64-bit browser is faster on a 64-bit system, but from what I see, the 32-bit versions are just as fast, if not faster... and consume less resources.
I guess it all depends upon what is meant by "faster." When it comes to javascript, then perhaps, indeed, 64-bit browsers are faster. However, as far as launching quickly and navigating the web, I see 32-bit versions as faster.
I think it also depends upon the specific system; 64-bit takes advantage of large RAM. If you do not have excess RAM capacity, then 64-bit browser is of no real benefit. However, this would mean your system has consumed almost all RAM.
For system with 4 GB or more RAM, I would at least try 64 bit.
You can always add SoftPerfect RAM Disk to speed up your browser (both 32 and 64 bit) by placing temp\cache files on RAM Disk. Plus there is added security advantage if you create RAM Disk that deletes everything upon reboot.
All in all there is no absolute need for 64 bit browser; as far as I know there certainly is no security advantage.