I've got them both and think very highly of them. I prefer the Stretch over most other models because the handle is designed so well that it fills my hands perfectly, giving a secure grip (or, at least, it feels more secure, which is good enough for me).
I like the XL because it's a reasonably large knife that is light, thin, discreet and perfectly capable. I was initially apprehensive about the lack of liners, but it really doesn't feel flimsy in use. I did not expect to enjoy the XL LW version as much as I do, I have carried it almost daily since I received it. FRN is good stuff.
That said, the regular Stretch 2 does feel a bit more solid (probably due to the extra weight from the liners) and is my go-to folder for when I anticipate rougher use.
Here they are in comparison to the Military 2 and PM2:
I will agree with earlier commentary in that I have learned to dislike forward choils. They reduce the length of the cutting edge and make the handle a bit awkward to me. If I'm in a rush, I don't want to have to think about how/where I'm going to hold the knife and fiddle around to get it right. I can, however, look past it for these models because they do everything else properly.
Truth be told, I seem to gravitate towards Spyderco's FRN models in almost every instance. If they were to offer any of the above models without the forward choil I think I would have to double my collection right away.
As for the steel options: I love K390 but prefer Cruwear/PD1 as it requires less maintenance. I have, however, been pleased with SPY27 so far and wouldn't hesitate to buy another knife made with it.
In short, you can't go wrong.