Hi there, welcome to the AT forums.
Cat5e is rated for gigabit ethernet. Cat5e afaik has not been rated for anything beyond GigE. Cat6 is also rated for gigabit ethernet. It is a higher quality cable, more bandwidth, and thicker physically so in terms of future growth Cat6 definitely has more headroom/options than Cat5e.
Cat5e walljacks, patch panels, and punchdown blocks are cheaper than Cat6 rated ones and for a smaller office GigE should be more than sufficient for quitte some time as long as you aren't in need of a LOT of bandwidth. If there isn't a "large" cost difference for wiring the place up I'd go for the Cat6 but if the difference is signifigant 5e is what we're running and it has worked great for quite a while.
Gaidin