Just received so haven't had a chance to test the K390, but heard it was pretty good. I'll talk about the knife and how it compares with the Stretch (drop spine, not the straight). I like the Stretch and blade shape quite a bit but thought i'd try the Endela. I've heard things like they were too similar, the Endela needed the choil (Spyderco fanatics), or the Endela didn't have as good of a handle.
The Stretch handle is comfortable and hand filling with a different shape. The Endela isn't a problem though and feels and fits just fine. It might even feel better for some who don't like too much of a handle...Endela sits in the pocket a little better too. Endela does add jimping on top of the FRN scales at the blade end the Stretch does not have.
The Stretch also feels much heavier for some reason. The additional girth of the handle must make that much of a difference. The Endela does not have a finger choil. Go Stretch if you have to have it. I like Sypdercos but i prefer their makes that do not have the choil. Reduces blade cutting length too much for me and if i need to choke up on the handle for control, i should have had a shorter blade anyway. Those choils also get hung up in soft materials like poly or fabric (or anything that can catch those damn choils) as i'm cutting, unless you always keep your finger in them.
I like this knife and will be using it more than the Stretch. I also prefer lockbacks for strength and simplicity and Spyderco does them well. I own a bunch of different knives with different locks but a good Iockback means a rugged and reliable everyday user you don't have to worry about. Also easier to use with gloves. Have several Spydercos with lockbacks but the Stretch and Endela are easily Spydie Flicked or Thumb Flicked. They're both pretty smooth.
I might report on the K390 at some point.