I have always promoted the 2" snub as the ultimate BUG. You can't beat 5 rounds of the new 135 +P load from speer, in a compact revolver.
However, I recently bought a Kel-Tec P-3AT find it is a darn neat little gun. 7 rounds of .380 is also a good choice for BUG purposes. Mine has been just as reliable as my revolver.
I think the .38 revolver is still more powerful, offers more penetration, and the speer round is guarenteed to open up even after going through thick clothing in the winter time. However the .380 is more concealable and offers two extra rounds, I also trust the .380 round to be effective in a situation requiring a BUG (i.e. worst case scenario). Bottom line, both are great choices and you can't go wrong with either.
Since you suggest you are considering a 2" snub and Sig 230, I would suggest the S&W revolver as I beleive it is more concealable in a pocket or on an ankle, two of the most popular places BUGs are kept.