ok the difference of these two bags are the 350 is a little bigger than the 250. the 350 is able to hold up to a 17" notebook and has a little more space to put your gadgets in.
ok technical specs per bag:
250: 1 digital SLR with a mid-range zoom lens attached plus 2-3 additional lenses or flash units (up to a 70-300 f/4.5, 15.4" widescreen notebook, memory cards, batteries and cables
350: 1 PRO digital SLR with lens attached (up to 70-200mm f/2.8 ), 2-3 additional lenses or flash units, 17" widescreen notebook, memory cards, batteries and cables
(pro is a little bigger than the regular slr if u didn't know:)
or go here: http://www.cameralabs.com/forum/viewtopic.php?t=10817
hope this helps