Not a dumb question at all.
The 7100 series will likely become a dinosaur post 8100. But that will take time, as the roll out for 8100 products across all carriers will be a year out at least.
There are some major differences between the 8100 and 7100 series, but the only thing that the 8100 series will lack will be a trackwheel. However, the trackball was designed to replace that. Will probably take some getting used to, but no more so than when we learned the wheel in the first place.
If you're toying with a decision between the two, the 8100 is the new hotness, and will probably end up being your choice. Any limitations it may or may not have will be carrier-related.
Hope that helps.