The big difference in feel between fluid and mag is that fluid trainers have progressive resistance, meaning the faster you go the harder it gets, like on the road. With most mag trainers the only way to increase is with a dial control. I say most because Cyclops has a mag model that has a progressive resistance device built in, but I have never tried it.

As indicated above, if you are just spinning the mag models are ok and economical. But for intervals a fluid model is best, and the KK is the only one with a completely sealed resistance unit.