The largest ETH heater is 300 watts. may not be quite enough for a 90-100 gallon. its enough for my 75gal in the winter in a drafty area though, so it might work on a 90 if the wall its on isnt' an exterior wall (usually a cold wall).
I'd run one, or even two canisters on a tank that size. Two if you plan to keep 'messy' fish, like cichlids, an oscar, goldies...any carnivores.
I run coralife digital thermometers on all my tanks, mainly because i got them for $10 at the LFS
...one of the few items he doesn't have to mark up a ton. Online I think they go for $6 though.