I've got a Sabre 30. Its a good simple daysack. The back system is ok and the straps are good, however, I find the size a little annoying at times. It's just, and I mean just, a little bit too small for anything more than a "day" sack i.e. something to take for a walk in the woods. Ahjno I'm impressed that you can fit that much in! By the time I've got my warm kit (buffalo type smock), a few id books, gaiters etc in it seems to fill up very rapidly. When the main sack is full it's also very hard to fit anything into the little "wand pockets" (I think thats what they call them!) on the side.
I've not used the Sabre 45 but I suspect that it may be a better choice. Again no experience with the Munro either so can't help there.
......... just to throw something else into the mix .....have you considered the Karrimor Delta 35? I've got the 25 and again find it an excellent little pack ..... but again I think the extra 10L would make a world of difference. The "wand pockets" on this are made of elasticated mesh and designed to hold the issue black water bottle ..... in fact they have little elastic toggles to go round the top of the bottle to keep them secure in the pocket. The back system is as good if not a little better than the Sabre 30/45. It is also small at the top and flares towards the base so everything sits low and secure on your hips giving a feeling of stability ........ and one of the most important parts ..... as it sits low on the back it's less likely to catch on branches when you duck under them! It has an outside map pocket and a small "key" pocket inside as well as a separate compartment inside for a water bladder ...... although I find it more useful for separating kit.
My only worry with the sack is that it is secured with a water resistant zip ..... I've never had any trouble with it and I've never heard of anyone having trouble with it but I can't shake the feeling that this is a point of weakness.
Anyway hope that was food for thought.
Here are a few pics of the Delta 25:-
Back system
Front showing "x" compression straps and map pocket (between the 2 light coloured strips), also elastic "net"
large "wand pockets" with elast toggle for securing top of water bottle
mesh belt
top of the sack with grab handle
access zip undone showing inside
key pocket
hydration bladder pocket