Hokay, logistically, Falmouth is the better option over Penzance. To launch from Mylor http://www.mylor.com/
gives a good slip, plenty of secure parking, trailer storage, fuel etc. However, you're almost doubling the length of the trip. Rounding Lizard point on a flood tide can be interesting.
Buying fuel on the Scillies is a pain, it's expensive, the pump is not often manned and it has to be carried in cans to the boats. It's best to carry enough to do the whole trip if poss. A round trip with some exploring around the islands usually totals about 200Nm, so fuel amounts can be calculated accordingly. We camp on Bryher which works well, they'll collect your gear with a tractor and trailer from the jetty and deliver it to the site. The pub is just down the track.