Which are better for reproducible power output: Mag- or Fluid-trainers? I'm looking to buy a trainer but my primary concern is being able to consistently improve my power output.  To do this I need a way to measure my output consistently - I don't care what the actual power output is, only that I'm consistently increasing it.  Given the linear speed/power curve of a mag trainer I would expect that as long as I leave the resistance setting alone my velocity is a good proxy for my power output.  I expect that with a fluid trainer that will be much harder to do because at the margin tiny differences in speed translate to much larger differences in power output.  Am I right?
The power-speed curve of trainers are subject to variations during a ride and from ride to ride depending on various factors including:


heat build up in the resistance unit and tyre
tyre pressure
press on force of the roller against the tyre
the tyre used, and 
wear and tear of the unit. 


Some of course use a direct drive rather than a roller pressed against the tyre.

Any of these things can and do affect the relationship between wheel speed and power demand. Some changes occur over minutes, and some occur over weeks, months and years.

What's more important than the shape of the resistance curve is having a trainer which provides:


a modestly realistic ride feel, which generally comes from having a trainer with higher inertial load (heavier/larger flywheel and/or one that rotates more quickly). Mag trainers often don't provide a realistic feel and as a result your ability to sustain power may be compromised. Everyone is different of course but in my experience I was able to hold far high power output on a quality high inertia trainer than I ever could on a low inertia mag resistance unit
consistently reproducible resistance from ride to ride, IOW the set up required for resistance to be consistent from day to day is easy to replicate
not subject to much heat based drift in resistance once an initial "warm-up" period has settled the trainer/tyre, and the warm up period is not long (e.g. 10-minutes). Some units are much better than others and some the resistance will increase with heat, others will decrease with heat
has a sufficient range of resistance load for your personal fitness and training needs


There are of course a whole range of other factors to consider with trainers (cost, robustness, ease of use, service and support, portability, size, weight, stability, noise, compatibility with your bikes, use with online and other software training aids etc etc) so I'm just talking about those relating to the nature of the resistance and ride feel.

To measure power output consistently though, you'd really need a good power meter. If you do have a good power meter, then of course the need for day to day consistency of the trainer power curve lessens somewhat as you'll have a better means to assess actual load. Even so, good ride feel is important and may make a difference to the power you are able to sustain.

If you have a reasonably predictable resistance trainer, then it will suffice for the purpose of assessing larger changes in fitness, but as fitness gains become more marginal (they do eventually), then the error in measurement means this method will no longer be suitable, and you'd need a better means of tracking fitness changes if that was important to you.

I can't specifically comment on individual trainers, but there are definitely trainers that are far more predictable in their resistance than others and also provide for a realistic ride feel. Examples of good trainers I am aware of include the Kurt Kinetic Road Machine and the old Lemond Revolution but there are no doubt others.

Of course the training you do doesn't need to be so precise wrt intensity, as being at the right overall level matters far more than being at a precise wattage.

Finally, while some trainer resistance curves are more linear and others are more curvilinear, I suspect that over narrow wattage bands, the curvature is not so high as to enable greater precision beyond those error factors I've mentioned.