I own a meter and a Cycleops Fluid 2. I use that setup at home.
At work, we work for a ****** Danish company that digs cycling, so the gym has Cycleops 400 Pro bikes with powertap.
So, here's my take on both having done my training rides in all three environments:
Outdoors:
I feel like this is the best way to do something like "peak and fade" intervals. I also feel it's the best for "endurance miles" with some steady state thrown in. Otherwise, try doing under/overs on the road. LOL. Unless you have a rock steady 15min climb, not happening.
The fluid trainer:
I got the setup cheap on Craigslist used. I got the trainer, mat, skewer, and climbing block for under $100. This setup at home I have to use a crap box fan. It's a "stopgap" workout method when I can't use the gym. The "road feel" is total crap, IMO. Doesn't feel realistic at all. But, I can memorize my workout or put it on a stickey note and watch the power/cadence and time and crank out a workout per the plan. Also, power on this setup is always easily down about 20w across the board versus outdoors or the gym.
The Cycleops 400 Pro:
For prescribed workouts, the best of all 3 for me. The only interval it's not good for is peak/fades. The big flywheel makes the road feel about 100% better than the fluid. Not perfect or ideal vs. an electronic direct drive, but better. I use the Rouvy/Cycleops app with it so I don't have to flip screens on the computer. I also like that I can torque that thing around or climb on it without worrying about destroying a carbon frame in a fluid trainer.
The only fault of the 400 Pro is that it doesn't have gears or a setup that you could really use for Zwift. The gears on your bike on the fluid are one plus over the 400 Pro. That thing does NOT like it if you rapidly change your cadence. It'll swing erg mode all over the place. Whereas a fluid you don't have to worry, you have gears.
I think the direct drive models out there using your own bike with gears is probably the best.