Hopefully you're going to try this bag out for size in the shop first!!!

I had the Slingshot 200 for a while, loved it, and then thought I needed more room and upgraded to the 300...............oops - big mistake.
Unlike the 200, the 300 doesn't come with a chest strap (unless Lowepro have recently revised the model) which means that in the normal rucksack carrying position it tends to hang awkwardly due to the weight of contents. The chest strap of the 200 counteracts this trend quite a bit. I also found it to be one of those.....how shall we say.....in between bags - neither small nor big. Too small for getting my entire kit into, yet too big when out and about hiking. Oh the 300 will pack a lot in all right and doesn't your shoulder know it after an hour or two. Ok, so lesson learnt - I sold the 300 on ebay for just a few Euros less than I bought it for and reverted back to the 200. In short, try the bag first - preferably with some weight in it before committing to purchase.
I doubt you'll find a photographer of long standing on this forum who hasn't gone through his or her fair share of bags over the years and still hasn't found the ideal........even if there is such a thing.