I use the Osprey Manta (20 I think) daily for commuting and also for trailriding and love it. The warranty is for real; I sent it back when a buckle broke (not sure if it was wear and tear or the car door that did it) and they repaired it and sent it back. No receipt, no time limit etc., it was just hard to part with it for a few weeks.
Did you check the measurement? I recall when I got the small, I measured from that big neckbone down to hipbone level based on info on their website. I was pretty close to the medium measurement, and I think either would have worked.
Personally I prefer the Manta's back panel, especially in hot weather - the mesh keeps it off your back and allows air flow. It's called the Airspeed design and I think it is available on other models too. I'm sure the Raven/Raptor design is better than most, but it doesn't look as ventilated. I also like the built-in rain cover the Manta has.