Awesome! I wasn't aware that there were actual usage stats on 7 vs. 8. By the looks of things, I think I'll target WP8. Of course, I may be using this as an excuse to get myself a new phone... :P
One of the features I will be using heavily is the mapping services. I understand that with WP8, they ditched the Bing maps control and replaced it with something considerably better. There's also the improved speed of the new GeoLocation functions, which would also be very nice to use.
In case anyone is wondering what I'm up to, I'm looking at making a Windows Phone client for Pylons (an online service I'm starting):
Pylons