I think either will be plenty fast enough, since most connections out to the ISP are limited to 1.5Mb anyway.
However, the plus in my decision for a Netgear RT311 was that I have a standalone, dedicated, less prone to failure, independant hardware device doing my NAT, and I don't have to worry about a PC gateway crashing, or going down for hardware maintenance/upgrades, or a slowdown/lockup due to a buggy Micro$oft OS (Sygate runs on Win9x/ME/NT/2K/XP).
The Netgear is dedicated to what it does, and it does it pretty darn well......