Both the 80 and 90 are good instruments. With regard to aperture, theoretically you have the extra 10mm. However, the 80 is a refractor, the 90 is a Maksutov-Cassegrain. There is a small secondary mirror built into the center of the front corrector plate. This does have an affect on overall light transmission, so with regard to light grasp, it's probably a wash or pretty dang close.

The 80 is an f/5 focal ratio, fairly fast and good for the Messier catalog and some of the other, relatively brighter, DSO's. The 90 is an f/13.8. Higher magnification due to the longer focal length, and higher contrast. Probably best suited for lunar/planetary work, although I've seen some decent AP's of clusters that are pinpoint sharp.

So depends on what you want to concentrate on. In actuality, both will probably do passably well on the majority of your basic observable stuff.

As far as the difference between the standard and backpack editions of the 80: the backpack edition comes with a backpack to stow the scope, and features a slightly lighter tripod (it does, however, have a hook underneath to hang a weight to improve stability if you desire.) The standard 80 has a somewhat beefier tripod and no backpack. Otherwise, near as I can tell, they are identical in every other respect.