Matt, how long are you going for? I went to Belize last year and spent 9 days on AC before heading to Guatemala for a few days and then on to San Ignacio, so I got to see a little bit of everything. I did a lot of research before going and found that a day trip to Tikal would cost me about $350-$400 US and would get me there for the hottest part of the day with alll the other tourists on their day tours. I found it to be much cheaper to spend a few days there and as a result, got to see much more than I would have by doing only a day trip from AC. The same applied to the tours I did from SI... A tour such as Xunantunich that would cost you about $180 US from AC runs about $40 from SI... or, you can do it even cheaper like I did, by joining up with a few other travellers once you get to SI and hire a taxi to take you there, and a guide to show you around once you arrive.
This year I'm only going for a week and will be spending most of my time on the island and maybe only doing one day trip. Since I'm so short on time, it's the best I can do, but if you have more time to spare and want to spend your money more wisely, I'd highly recommend staying closer to some of the sites you want to see so you can get a fuller (not to mention more leisurely) experience.