My 2 cents:
I think a logic analyzer is better for digital stuff. Scope lets you look at 2 signals, logic analylzer is much better.
At the same time: almost all the "design" issues seen in the forum are software oriented - and serial.print is about all that is needed.
Everything else is misconnected hardware, and getting someone to actually post their design usually takes care of that.
Everything coming out of the uC is digital.
Maybe some analog stuff goes on, a scope is good for that.
I have a USB scope and really haven't used it since figuring out a TTL serial signal was in-active low vs high.
I bought a Saleae 8 channel logic analyzer for $150 and have not used it once.
A little bit of care in the code and you can do just about anything.
If you're tweaking PWM outputs, a scope could be handy for confirming things.
Playing with analog inputs, scope could be handy.