Personally, I wouldn't spend any money on a single channel scope of any type for digital work.
You'll find very quickly that you'll be wanting to time one signal vs another (think encoder transition or SPI connection) and you can't do that with a single channel scope.
You can find very low price two channel scopes on the world inter web for about that much with a third trigger input and much higher bandwidth.
If I was on a tight budget, I would personally buy and deal with inconvenience of 2 channel USB scope before buying a portable single channel scope.
I have a 2 channel Fluke Scopemeter I got for $275 on ebay. It took some time to find it (weeks) but it was an economical solution.
General rules about scopes:
MS/s is how many times it samples per second. It is the digital speed of the scope. If you care about shape of signal, you'll want 5x to 10x sample rate from signal. 40 MS/s scope will display 4 MHz signal clearly. At 8 Mhz, a 40 MS/s scope will start to show distortion that can make timing of signals tricky.
Bandwidth (MHz) is a function of the analog input to the scope. You want 2x to 4x MHz on scope vs analog signal. For analog signals, a lower bandwidth results in distortion changing the shape of the signal, and in digital system, bandwidth restrictions round off the sharp edges, again making timing tricky. Remember that even a low frequency digital signal has a very high frequency content, but unless you're trouble shooting communication and clock lines, you don't need the scope to measure the high frequency stuff.
For your use case, you likely will not run into any issues with the limited bandwidth and sampling of cheap scopes. A cheap scope will be plenty, and for home robotics, a cheap 2 channel scope is 10 x more useful than the best single channel scope.