Just to dredge this topic up from the dead.
I had a play with both the small and the pro in Merchant City Cameras (who incidentally were a breath of fresh air to deal with) and decided to go with the small. Chose to go for it in black, even though I'd originally intended to go with tan. Upon seeing the tan it looked a lot more 'on safari' than the retro military bag I had expected. Although the pro provided plenty of (kit) growing room it was just far too big, my kit looked lost in it. I also reckoned the small looked much better without the top carry handle. Truth be told if there was an inbetween size I'd have gone for that one, but as it is the small fits my needs perfectly right now. Finally I can just pickup my bag and get out with my camera rather than looking out the window and wondering what the weather's going to do.
And hey, if my collection of kit grows and I feel the need to carry more around with me I've got the perfect excuse to go buy the pro!