I've had Pandemic and On The Brink for a while, recently we tried giving the Bio-Terrorist challenge a go. We weren't sure how many epidemic cards to use.
For the standard game (no purple cubes but using the role cards from On The Brink) we usually use 5 epidemics and with experienced players win probably about half the time. As the Bio-Terrorist I suggested using 5 epidemics, but the other 2 players wanted to use 4 so we went with that. In the end I won quite early on when the first epidemic card caused a massive chain-reaction.
So, is the Bio-Terrorist variant harder than the standard game? The other players thought so, as you have a 5th disease to deal with. I'm not so sure as the purple disease is quite easy to cure and the cards that the Bio-Terrorist draws and discards ought to lessen the impact of epidemics.