Welcome! Glad you're de-lurking!
I can see why the backstitch would be more sturdy, especially on a
non-felted bag. There will be less vertical stretch, which is great when you put stuff in the purse. (I made one purse that stretched to my knees when filled! LOL) Anyway, I'd recommend doing the backstitch.
I've never seamed anything with slipped stitch edges. I only do that when I'm not going to seam. But I don't think it will pose that much of a problem. Just make sure your stitches are close enough to avoid gaps.