A supercharger is upgradeable in the sense that you can change the pulley size and thus increase the boost (this is how you change PSI on a supercharger, by getting a smaller pulley). Also for example the Comptech Superchager kit for the RSX Type-S and soon the '06 Civic Si comes with an ECU reflash from Hondata which tunes the ECU specifically for the added supercharger. Jackson Racing has this same sort of thing but only for it's "race" supercharger, and not the "street" one. So yes, Superchargers are "tuneable" but only from an ECU perspective in order to tune the ECU to take full advantage of the charger.
As for the debate, you have to ask yourself what you're looking for. Both are completely different and take you down different paths. If you just want a dyno monster and want to push insane amounts of HP then you're only bet is a turbo. Superchargers only go so far in terms of performance, but they are extremely linear (just imagine pushing the HP and Torque curves straight up by about 40-50+ HP). Turbos are another story and you'll obviously have the lag and thus only see the gains after it is spooled up. IMO I'm going for a supercharger since its zero lag, quite easy to maintain, and gives you lots of torque on hand from the get go. Plus by time you add a supercharger and I/H/E combo, you'll be pushing somewhere upwards of 250+ WHP on the Si. Remember that 300WHP is about the max you'd ever want to have for a front wheel drive car.
Neither is better than the other, each has advantages that the other doesn't have. Period.