I'm coming from a White Lumia 900 with 7.5 on it and if I were in your boat, I'd probably wait. I mean yeah WP8 has a lot of very cool things going on but the 900 is a great phone. I love mine. When 7.8 hits the street (which could be soon if you believe the rumors) it'll give the 900 a lot of the same interface features. With all the goodies that WP8 has, it's the small tiles on the start screen that I think I love the most (which 7.8 will give you). Ultimately it's your decision. I got my Yellow 920 with a contract. If I hadn't been able to do that I'd have waited.
-joe-