you can go way harder on a trainer than on rollers. you can also
safely test extreme high cadence allowing your form to take a
back seat while you probe you physical limits (first time you
spin 198 cadence is an eye-opener. I lose 'roller form' above 160)

rollers teach peloton kung-fu and technique, which is crucial
to ride in fast groups. they are only hard the first few times, then
it become easier, yet you can never lose focus. you can work up
to no-handed water bottle swapping then one legged spinning
on rollers, no handed, then I'd say you might have good form to
stay in a peloton at speed (tho' shoulder and handlebar bumping
is something you can only learn on the road) don't try this at home
folks...it is fun to test yerself but not necessary to become a
cycling guru (which i am not)

stationaries can make you see stars and come close to passing out
which you cannot always do on rollers without coming off them by accident.

you can come close, but no dice on the passing out part. which is part of extreme
anaerobic training if you want to test your limits and train your body to
hit that spot, and back off and recover to BLT levels without having
to stop. no one needs to pass out on a bike, but it sure helps to come close to it
then you can really meter yourself in real world hammering sessions and not
blow up. last time I passed out on the road, it sucked. I fell off and it hurt.

on the stationary, hitting 209bpm and the tunnel vision started,
I just leaned over the bars and took a 5 second forced nap, then
continued to spin easy. on rollers I'd have smashed up my room

survey says, get both.

rollers and a stationary setup. for the unique
aspects of each, and it is never boring when you swap out spinervals
dvd's and switch up trainers ....it doubles the action during the winter