I've got a Classic 32 and an Ultra 12. I've had the classic 32 for 3 years or so, and it's done many hundreds of miles at running pace rammed with stuff. It's starting to get a bit battered and worn now, but it's had hard treatment for a good long while. I've fallen on it, fallen in peat bogs with it, sweated unbelievable amounts into it, massively overloaded it and it's still good. The PU lining is starting to wear away from the nylon inside, but it never was all that waterproof anyway. Carrying a 700ml water bottle in the side mesh pocket for most of the miles has started to stretch/wear a hole, but that's only just happening now. They're a *lot* more hard wearing than they look.
The +10 is usually overload - if you let out all the straps to the maximum and loosen the straps holding the lid pocket down, that's how much extra you'll get in. "The neck collar of the sack is very generous" - so unrolling that will be part of the extra 10. 45 will be the total including the lid pocket, but these things tend not to be very precise. My Pod rocksac is theoretically 35l, but I'm pretty sure it's a lot more than 3l more than the OMM 32. OMM measurements for capacity are on the tight side, whereas Pod are on the generous side.
As far as size/fit goes, I'm 6'2 and pretty wiry, and the OMM 32l pack fits me fine. Pod Rocksac size 2 is the right size for me as well. That said, there'll be hundreds of people of all shapes and sizes running around the Howgills with them this weekend (I hope they're having fun, sadly injury stops me from joining them :-(). I'd hesitate to spend much on a pack without trying exactly that model for fit though. Of course you can always return it to the sender if you find it doesn't fit well.
Generally for 'fast and light' like running a multi day route, a long adventure race or something like that, I'd go for the OMM pack. I've had other ultra lightweight packs and they've been much less hard wearing. That said, an ultralightweight pack is designed to have a limited life - they're not going to be the pack that you batter, mistreat and love for 20 years. The Pod packs on the other hand, are bombproof (but much heavier).
Edit: Try running shops - 'Up and Running' sell OMM packs in various sizes (mostly the smaller ones, but they might be able to get a 45l in for you to try).
