I own these shoes and also a new pair of Avia 9997 shoes and compare them here. The Skechers have a soft "Kinetic Wedge" that extends from the heel to the front of the shoe so the rolling effect is very even from the heel to the front. The Avia has a distinct rocking point that is midway of the sole, pretty much under the arch.

The other thing about the Avia shoes is that they have a tough flexible plastic "energy wedge" [my term] that is embedded in the heel. These are supposed to absorb the heel-strike energy and then help the heel rebound by returning the energy as the heels lift off.

Avia calls this it's "Shear Cantilever System" and it may allow these shoes' heels to last longer than the Skechers because all the Skechers have is the outsole and then the pillow soft Kinetic Wedge. It's my impression that the softer Kinetic Wedge in the heel will break down faster than the tough-looking Avia Shear Cantilever system but this is just an impression.

The Skechers is very comfortable from the outset while the Avia is more ridged feeling (more stable actually) so the Avia rocking motion is very "directed" forward while the Skechers is less fussy about which direction you move (including side-to-side).

The other thing about the Shape-Ups XT is that the width of the outsole, moving forward towards the front of the shoe, is about 1/4" narrower than the Avia. And the Avia tread is more aggressive, and is broken by areas that allow the foot to flex a bit more as it travels forward.

Also:

(1) Skechers are distinctly heavier than the Avia's.

(2) The Skechers nylon loop lacing system has broken on me when I tried a pair of these in a store. I have read one other Skechers review where this happened, where one of the nylon lace-holding loops came loose from the shoe. The Avia's use the traditional lacing holes instead. So I can only conclude that the Avia's are longer-lasting in this area.

Avia's wider heel and deeper tread will probably translate to a more stable footing for trail hiking for people who are unsure that a Skecher could handle a trail or two.

So the differences now that I have them both is that the pressure hits you differently when you place your foot down. The Skechers feel like there is more pressure on the heel whereas the Avia's almost immediately feels support spread over more of my arch. The Skechers also feel less stable when I stand on them (they are in fact known as "unstable rocker sole" shoes like the MBTs), whereas the Avia's feel more stable; even though they have a raised pivot point in the middle of the sole that actually causes a little instability when you place them on flat surface. I.e. the shoes don't sit flat on the floor and even slightly wobble laterally left to right. In practice, I don't notice this lateral wobbling much when I am wearing them.

When simply standing, the Avia support feels more centered (bunched) under the arch whereas the Skechers feels like the support starts more towards the heel and extends forward towards the middle of the arch. This feeling is not right or wrong, just different.

So far, I would certainly prefer the Avia's on a trail than the Skechers but will update this review as I use the Skechers more.

8-18-11 Update: Still loving both the shoes; Preferring the Avias to the Skechers for walking because they are lighter and more stable. The Skechers are much easier to put on because they don't cup the heel so tightly and they are softer inside, so I still like them but not for a full day of work and walking because their heel is too unstable. In fact, I almost twisted my ankle when walking down twisting stairs and the soft narrow heel didn't keep my foot from twisting when I was turning. So I am more careful when walking with these than the Avias.