i have an analog 20 Mhz osciloscope for 8051 it was enough for me i never needed more power

20MHz = 50ns. I have seen noise pulses triggering events down to 10ns. I even, once had to reason my way to a glitch that even a 100 MHz scope could not catch. It took me a good while to start reasoning that something had to happen the scope could not catch.

If everything runs well and nothing strange happens and you use an old slow derivative, you may be able to get by with less than 100 MHz, but the real problem is that a scope that does not show all will sucker you into believing that it does. And THAT will get you.

More than half the apps I have worked on has had signals that occured too rarely to be seen without storage, thus that recommendation.

Erik