The reason I went with the larger supercharger is, so I could make the same or more boost and not heat up the charge as much. This means more power and less chance of detonation.
The smaller the blower the faster you have to spin it to make the same boost as a larger blower. The faster you spin them the more heat they make and heat up the intake charge. When the charge is heated it doesn't make as much power and has more tendancy to detonate. With my 8-71 I can make 12lbs of boost at 10% underdriven (blower turning 10% slower than the crankshaft). With a 6-71 I would have to probably run it slightly overdriven to accomplish the same thing. This is all done by the pulleys you choose. A 6-71 is usually plenty for most small blocks and mild big blocks. The 8-71 was a little more money up front but, it leaves room to grow as well. My goal was to make as much power as possible on pump gas so the 8-71 was right for me.
Sorry for the long winded response, I just like to make sure things are clear.