I didn't get a chance to respond earlier on this thread but it looks like you are coming around to a great decision one way or another!
Its awesome that you were able to have an open conversation with your friend. I'm sure it would be sad for her if you just started buying with someone else without saying anything to her beforehand; but by exploring options with her - especially for signing up yourself, it lets her be a part of you getting the best option for you without cutting her out entirely.
I bet it will be a lot of fun for you to both be demos and friends. I convinced my mom to sign up as a demo when I was still telling myself there was no way I could do it. But after seeing what information and perks she got, and being able to talk with her about it, I ended up signing up quickly after. Now, it is a lot of fun to be able to hang out, craft, talk about the latest SU and craft world happenings. Hopefully you two will have the same thing!