I have the OM 28/2.8 and the 28/3.5
Both are wonderful but I like the 2.8 just a little better. It's easier to see through and there is that extra stop (or 2/3 stop).
The price between them is not so great if you shop and wait for a good one to come to you.
I would choose on condition more than anything else. Often you can find a 3.5 still with its case and lens hood for a very good price.
Most people don't know this but when new, Olympus sold these lenses complete with the leather case, the lens hood and the caps.
Slimey resellers will break them up and sell all items separately. I think that stinks but then most sellers don't give a crap they are simply greedy.
So... look for one that is complete and sold by an owner or his kids.