to be honest mate, on the battle field, you might as well be asking which is bettter, halberdiers or cannons.
they both have very different roles, outriders are there to cover flanks and provide fire support. pistoliers are there to hunt WM crew, re-direct and harrass blocks. totally different!
if you are already taking BS fire you'll prob get more out of pistoliers as they'll give you functionality you didnt previously have