Why not do both!!?
The old Lancia S4 Group B rally car had a supercharger for instant throttle response and when a certain amount of boost (About 1 bar I think) was reached the turbo took over to about 1.6 bar.
Ive recently seen a MK1 Fiesta with this done, a home done affair, compleate with nitrous! I think it kicked out about 370bhp!
Your point about retaining standard compression is fair enough but it does limit the amount of boost you could run.
I think the important point in all of this is the control of the system and the mating of the indiviual componants. What I mean is that for a given engine you could run three different turbo's for eg Garret T3, T31/2 and a T4. The T3 would be responsive but not be able to give the ultimate power, the T31/2 would give a good balance, whilst the T4 would give big power but more throttle lag.
If I were doing the conversion I'd go for the turbo because of cost and availability. Dont get me wrong superchargers are excellent.
I'd try and graft an existing system onto the alfa lump...Take a full Cossy injection and turbo system and put it onto the 155. Swap all the temp sensors, map and throttle sensors and drop the compression to similar to that of the cozzy (8.5:1?) et al. You'd have to make up several bits, llike crank timming points. For all intense and purposes the injection system would think its on a Ford! Not easy but anyone who owns an Alfa must be mechanicaly minded!