The Spider Pen Pro and the Tarantula are designed completely differently.
However, both will execute the same effects.
Both are reels.
SPP is motorized while T2 is not (ITR)
The SPP can be in plain sight - it's a pen.
T2 needs to be hidden - again, it's an ITR.
There, that's a start.
Oh yea, not to overlook - approx $20.00 difference in cost.
Others can fill in the rest.