does anybody know if they're different and will it be worth it?

Worth what ???
Fitting the SP to an RR ... not really.

But fitting the 2TK to an 3en is as it's an unrestricted cdi (no 180kmh speed restriction), the only thing is the M-Max box would only cost £15-20 second hand whereas the cdi will be about £35-50.

The money can be better spent on tyres suspension or brakes ... or lower the gearing.