I just picked up a nice used surplus LBT 2657 "8-pocket" ruck on ebay at a pretty decent price, especially compared to new retail on MR and TT. What I wanted was basically an updated large ALICE in OCP. So it's a nice middle ground price-wise between surplus ALICE, and the newer modified packs. If cost is a factor, then you can save a lot going this route, or be able to buy a couple for the price of one other ruck.
On the medium, if the specops is no longer around, you can still take it in to one of the shops and get the same mods done on it as the large rucks. There's still some room on it for a few extra pouches.
I just picked up another Down East frame with their "mountain" style suspension. This would be a good upgrade on any ALICE compatible bag. It gives you a suspension very much like the "Futura" style on the MR rucks. The suspension harness is an adjustable "Y" that runs down your back, with top load stabilizer straps where the shoulder straps normally attach. You might find this more comfortable on a long 4 1/2 hour ruck like that. I like this style a lot, because the weight pulls from your mid back, and completely over your shoulders, instead of just pulling from the mid-point, or top of your shoulders. The ALICE style attach points places all the weight and pressure on the top-front part of your shoulders. The other way is more evenly load, as long as the top stab straps hold the ruck in close to your back. Obviously, without them the ruck swings way out from the back, negating any balance to the system.
Back to the OP, my opinion is a large ALICE style bag would be the best all-around pack for an ALICE style frame. The mediums aren't much larger than the 3-day assault packs, with hardly any room for a cold weather sleep system. Of these, TT, MR, and LBT are the main contenders, with surplus LBT being the best bargain right now.