the difference between a 11/25 and 11/32 will be that you will have more gears for climbing with the 32.
My suggestion is to get the same gearing as you have on your real bike, that will make riding the trainer similar to your real bike.
If you go for the 11/25 and you feel the mountains in zwift is to hard on your knees then you can always adjust the trainer difficulty down in the zwift settings.
@James_Grieve
To back up what Gerrie said - the cassette size range doesn’t matter to Zwift. It matters for compatibility with your bike’s drivetrain, in particular what your rear derailleur can handle
It’s best to match the cassette you install on the trainer to the cassette that’s on your rear wheel.
Derailleurs have a limit on the largest rear cog it can handle. Go too big, and shifting on the climbing end of the gears will suffer, or possibly, not shift at all.
If it was installed correctly, the chain on your bike is also cut and sized to the largest rear cog on your wheel. Putting a too-large or too-small cassette will negatively affect your rear shifting because the chain is now too short / long relative to the rear wheel.