Very interesting indeed, the Sherpa's I know are the trials machines, so very grunty engine with massive torque but not fast, (my 350cc can theoretical go 50 miles/h which I will never try out). For the city this is OK, certainly an other secondary gear ratio will help very much.
On the other hand, very light and you don't sit really uncomfortable may be another foot peg location will help too.
BUT as the Alpina was made as an demand from US I would take an Alpina model, better seat position, already changed foot peg position, gear ratios are much better and secondary gear ratio too.
An Alpina model I would recommend you, as these must be too more often in craiglist ad's in the US then the Sherpas.
To the interchangeability be careful the engine of the Alpina and the Sherpa are for example completly different, (it looks that the engine are clones but they have different displacement, gear ratios, even the engine side cases on both bikes are in detail different!)
Looking forward to pic's.