Here's a recent article on ghacks comparing performance between 32 and 64 bit browsers. Many interesting and informative charts and graphs. I pasted an example of ram usage comparisons in the 2 versions of Firefox. http://www.ghacks.net/2016/01/03/32-bit-vs-64-bit-browsers-which-version-has-the-edge/
Here's a reason to consider using a 64 bit browser: https://threatpost.com/latest-emet-bypass-targets-wow64-windows-subsystem/115224/
32-bit here because the memory use became extremely high and caused freezes with 64-bit on my old system (3 GB RAM).
32 bit. I find the 64 bit uses twice the memory and chokes a little on sites that seem fine on 32 bit. That, and the obvious compatibility issues. I really wanted a 64 bit but unlike everyone else they haven't gotten it right. 64 bit Chrome seems to have no such issues.