If you ride with a group and you've just started, get the rollers. They build form quickly. Even after many years of riding/racing, if I feel like my form is terrible (again) I'll get on rollers a couple times. I should probably do it now but I'm being lazy.
Trainers are great for overall fitness. I use the trainer year round, meaning for various reasons I can't or won't go outside and train. You can do easy rides, hard rides, intervals (all but sprinting basically).
Specialty devices:
- Rock N Road trainer (Kurt) - allows you to stand and rock the bike
- e-motion roller (
link within BF) - these allow you to (pretty easily) stand while on rollers
As a regular indoor rider I built a set of free motion rollers. I think my long term habits from riding regular rollers won't let me go full out on the free motion rollers. I rode them a few times but haven't ridden them in a while, maybe a year. They do allow you to get away with some pedaling habits that regular rollers strongly discourage.
I designed a conversion to a spare Cyclops frame I have to convert it to a home-made Rock N Road type rocking trainer. I don't weld (or use a plasma cutter - I could only find ~3/8" plate steel for free) but I haven't picked up the trainer yet. This thread it motivating me a bit as the trainer was done about 8 months ago.
If you buy a Fluid trainer you should get the Kurt. The CycleOps will leak, eventually. I have two right now, the third one leaked (hence I have an extra CycleOps frame). The Kurt fluid trainers don't have a shaft connecting the fluid chamber to the tire roller - they use magnets so the fluid is in its own sealed chamber, no shaft seal necessary.
Finally if you want to be able to fine tune your workouts in small increments (so, for example, on the road you usually shift one gear at a time, not 5 gears at a time), then you may want to consider the noisier mag trainer units. They do NOT have exponential resistance like fluid and wind trainers so you can step up your resistance a little at a time. If/when my next Fluid resistance unit goes I'm going to look for a mag unit for that reason. Mag units can give you a workout closer to motor pacing, where you can vary speed slightly without majorly influencing resistance. It's a minor thing but if I am motivated it's hard to find and maintain the sweet spot for resistance on a fluid trainer.
My tips for getting trainers - look in Craigslist or some local version of it. I bought a second set of rollers for cheap (I was going to use them as regular rollers but ended up loaning them to a teammate) and I regularly see other trainers listed.
The answer to the original question of which to get is "Get both if you can".