I think Idnan must be comparing a 220GSi with a 216 GTi.
I would say they are pretty much the same car in different trim. Same engine, same power,same gearbox, same chassis. Suspension is also the same although the GTi's sometimes get lowered more when aftermarket parts are added.
As far as I could make out the GTI has only 3 doors and slightly different bodykit. The GSi is as quick and a fair bit cheaper to insure. When I was looking there were about 3 groups difference between them!