Well, Tokyo is a huge, busy city with tons of people and lots going on. It also has the benefit of being fairly central to Japan, meaning that it's going to be a heck of a lot easier for you to visit other places in Japan without it becoming a huge hassle. Easy day trips from Tokyo can include places like Yokohama, Kamakura, and Nikko. If you're willing to go about 2-3 hours by shinkansen in any direction from Tokyo you can also get to places like Sendai, Kyoto, Nara, Osaka, Kobe, and Himeji, which could all be good weekend or break trips. Sapporo is pretty far removed from most of that (e.g. if you even wanted to visit Tokyo for a weekend it would be a heck of a trip) and it can get extremely cold and snowy up there in the winter. Personally, my vote would be for Tokyo just based on how convenient the location is, unless you REALLY hate cities (in which case I'm not sure that going to any of Japan's major cities would be a great idea) or REALLY love winter, nature, etc.