join vs. IN statement

We are currently trying to determine if their is a performance difference with postgres in using an IN statement in the where clause vs. using a join between tables.
Is there any published documentation or someone with personal experience that can answer this question? We are adding sessioning to our web interface and plan to append security info to the queries run from the interface.

I can add something like: ....... AND WHERE user IN (group1, group2)

or I can do something like ....... SELECT data FROM tables WHERE group_ID(of one table) = group_ID(of another table).

Any thoughts?
thanks
Jodi Kanter

_______________________________
Jodi L Kanter
BioInformatics Database Administrator
University of Virginia
(434) 924-2846
jkanter(at)virginia(dot)edu