DOT 5.1
advantages
high performance, higher boiling point either dry or wet than DOT 3 and DOT 4, (act boil pt is around 275*C dry, 200*C wet)
compatible with all rubber formulations
disad's:
non silicone, will absorb water (which could almost be thought of as an advantage because the silicone DOT 5 fluid puddles water in one place since it does not absorb water... no puddles with 5.1)(of course any water is bad because it causes corrosion.. )
will eat paint, (same as DOT 3 & 4, so nothing out of the ordinary)
difficult to find and expensive, usually only found at speed shops
I'd go with DOT 5.1, seems like it's better for performance applications and should work with DOT 4 if you need some fluid in a hurry due to being a non-silicone.
Silicone seems like high maintenance, hard to find, and doesn't mix with other brake fluid. Also gives a softer pedal feel since it's compressible, and has lower boiling point than DOT 4
EDIT: after re- reading the thread..
DOT 5 and DOT 5.1 are two seperate kinds of fluids..
DOT 5 is silicone, he is asking about 5.1, which is non-silicone.
DOT 5.1 has nothing to do with DOT 5, DOT 5.1 is like DOT 3 and DOT 4.. DOT 5 is not compatible with any of the rest since it is silicone.