You may not be able to increase performance via the carburetor...

It all depends on whether the carb is what's limiting performance. A well set-up Weber DGV is perfectly capable of feeding a stock B18 or a mild B20, and you probably won't see any real performance gains from SUs in daily driving. There are several drawbacks to the DGV for performance use, namely the small size of the venturis and the curly intake manifold with unequal runner lengths, but on the good side, it can be precisely tuned if you go through the trouble to do it right.

SUs offer a much straighter shot at the intake valves, and you get two 44mm throttles instead of 32mm plus 36mm. The piston/dome affair is a variable venturi, so airflow speeds across the jets remain fairly constant regardless of airflow volume. The main downside is their very simplicity -- there is not much chance of fine tuning them, and they are also sensitive to changes in barometric pressure. They can work very well in "approximately right" mode, and they will support more HP if you have a motor capable of generating more HP.

Your choice is between the HS6 and HI-F models of SU. The HI-F supposedly outperforms the HS6 at high RPMs, but I doubt you'd notice the difference in daily use. Condition is much more important that which model you get.