When shopping for your first strap-on, what are the things that you think are the most important to look for first? Right now I'm browsing around and getting ideas for my lady and I. I've decided that first of all, I want it to be comfortable and obviously adjustable. I'm mostly looking at the sets/kits.

And I see a few panty-harnesses or others that I like really well, but I see that they essentially fit like a thong in the back (obviously not double-strap) and I just can't see how that would be comfortable. So, I suppose my question for that would be.... would it just be too much or weird to wear my own undies underneath those type of harnesses if wearing alone was too uncomfortable for me?

I have a pretty well-rounded knowledge of sex toys in general, but I know virtually nothing about strap-ons. It would be both hers and my first strap-on.