If you had to pick one, then a supercharger would be the easyer to fit, like pgarner said, but in all fairness, the power increase would not be too great. If I was you, I'd look at upgrading the basic stuff. If you have the disposable cash to do so, then look at the supercharger, but It may cause your engine to overheat. In that case, you would have to sort out the cooling or add an intercooler, and if you had to do that, then it would have been more worth while installing a turbo. Its a catch 22 if you ask me. After the turbo, its a remap, then exhaust (straight through with dcat) and maby change of cams. Then you would be looking at the best end of 150-165bhp. But saying that, I don't know what sort of gear box are in the yaris, and wether the clutch can take it. Im sure it could.
Guy's disagree with me if im wrong.
I just don't know if it would be worth it. I mean, if your planning on selling in the next 2-3 years then you would get nothing back on what you have done to it.