Forget it. I used a BitScope USB-based scope for a while and it just doesn't cut it. A scope needs to be a standalone piece of test equipment, and is probably the most useful thing you'll ever buy so get the best one you can afford.
Don't bother with any scope of bandwidth <100MHz. You'll just be fooling yourself on most signals. I recommend the low-end Instek scopes (e.g., GDS-2102). It's ~$1100 if you shop around but it will last years, while a USB-based scope will just frustrate you.