Trainer shopping peaks in September and October, and the specification sheets are unhelpful because they lead with the numbers that differ least between products.
The three types
Wheel-on. Your rear wheel sits against a roller, which makes this the cheapest option and the quickest to fit and remove, though it is noisier. It wears your tyre, and power figures drift as the tyre heats and pressure changes, so it needs a spin-down calibration fairly regularly. Fine for a rider training a few hours a week who needs the space back afterwards.
Direct drive. You take the rear wheel off and mount the bike on the trainer's own cassette, which is quieter and more accurate, feels closer to the road, wears no tyre and generally needs no calibration. It costs more and is heavier to move. This is what most riders training seriously indoors end up with.
Smart bike. A complete stationary bike with adjustable geometry, which is expensive and excellent, and worth it only if the bike lives up permanently or several people use it.
The specifications that actually matter
Maximum resistance is quoted in watts, usually at a given speed, and most riders never approach it. Unless you sprint at very high power, it is not the deciding number.
Simulated gradient has the same problem, because a trainer that simulates twenty per cent is quoting a figure you will encounter rarely and will mostly ride in ERG anyway.
Flywheel weight matters more than either, because a heavier flywheel carries momentum, which makes the pedal stroke feel less like pushing against a dead stop. It contributes more than anything else to whether a trainer feels like riding.
Accuracy is quoted as a percentage, commonly between one and three, and it is worth less than it looks.
Noise matters more than people expect, and direct drive is dramatically quieter, so if you train early or late in a shared home this may decide the purchase for you.
Repeatability beats accuracy
Accuracy is how close the trainer is to true watts, and repeatability is whether it gives you the same reading for the same effort tomorrow.
For training, repeatability is what counts. If your trainer reads three per cent low but reads three per cent low every time, then your FTP test, your zones and every workout are all scaled against the same reference, so your training is correct.
Accuracy matters when you compare your figures with someone else's, or when you move between a power meter outdoors and a trainer indoors and want one set of zones. If you mainly ride indoors in winter and test indoors too, a very accurate trainer buys you less than the specification suggests.
Calibration, briefly
Wheel-on trainers need a spin-down after they have warmed up, and they are sensitive to tyre pressure and to how tightly the roller is pressed against the tyre, so set the pressure the same way every time and calibrate weekly.
Most direct-drive trainers self-calibrate and need nothing, though some ask for an occasional spin-down. Follow the manufacturer rather than a forum.
If your numbers move suddenly and your legs feel the same, suspect calibration before you suspect fitness.
What you also need
A fan will do more for your winter than the upgrade from a mid-range trainer to an expensive one, so budget for it before anything else.
You also want a mat for sweat and noise, something to raise the front wheel so the bike sits level, and a way to see the workout. WattPlan connects to Bluetooth trainers directly, along with a heart-rate strap and a power meter, so a phone or tablet propped in front of you is enough.
What to buy
If you are riding indoors a few hours a week and the bike comes down afterwards, a wheel-on trainer and a good fan will do the job, and the money you save is better spent on the fan than on the trainer.
If you are training indoors seriously through winter, or you ride early and late in a house with other people in it, direct drive is worth the difference, mostly for the quiet and the feel rather than for the accuracy figure.