I use fixed spreads,
just because it was easyer to program in my ea's,
also havent gotten signs of any severe slippage.
It also suits my trading style the best.
The choice is personal.
as long as i'm getting profits and get payed out I'm not complaining.
Opened an account through a rebate IB which gives me part of the spread.
So my opinion is fixed spread all the way :)
I am the change in the market that causes you to lose :p / Watch out before I negative pip you! ^^