Just my two cents as I've never owned a nice pack but am doing my own research as well. I'll also admit to being a little bit of a Sitka fanboy. That said...
If I was you, I'd get the MR. The MR would allow you to switch bags up without having to buy a new frame since they have a frame system. MR also has a long track record with their packs, and while Sitka has a phenomenal track record for quality and design in clothing, packs just aren't their bread and butter like they are with MR. Frankly, I was surprised when Sitka started licensing their Subalpine design to MR since they are clearly competitors in the pack space - but I think that speaks to Sitka's opinion of MR's packs, including the Metcalf.
Again, just my two cents. I'm sure the Sitka pack is great too, and there is a review on this site for it if you are interested.