Fluid trainers will get progressively harder as you increase effort. Mag trainers resistance are set and must be controlled via a cable. Both are interminably boring.
Best bang for the buck, IMO , is the Elite Qubo Interactive trainer (NOT the Smart B version). Lowest cost Smart trainer on the market. Can be used with Zwift, TrainerRoad, etc. Has reasonable "road feel" but doesn't simulate very steep pitches. I had one that I used for about 1.5 years....for the $$, it can't be beat.
$339 at Nashbar.
https://www.bikenashbar.com/cycling/elite-qubo-digital-interactive-trainer-el-qbodi-base |