You really need to think about what you need a 20 mhz scope is not much good on a 16 mhz uC
you need a 40 mhz.
I would go for a good 8 channel logic tool way more useful.
And a sound card scope is fun to play with and look at sound but it's too slow to be of much good.
There are good usb scopes but I would go for one that's faster and one that's for logic with 8 channels.