What would make you change signature campaigns?
Things that are currently in place:
Highest rates
0.1BTC x 10 bonuses to highest posters
Monthly payments
Payments made to addresses from user BIT-X.com accounts
Well personally I'd only change if PrimeDice ceased offering their deal, but I'd go for the highest paying though there are other caveats. I think people like bitmixer because of the following:
Automated payments
Weekly payments
No minimum posting requirement
Simple sign up
Reliable/trustworthy
Even if campaigns pay a little more than your one people would likely stick with it because of the reasons above so I think you'd be better off trying to find a unique angle.