Go mag if you want variable resistance, fluid otherwise. Variable resistance can be quite handy if you want to do high power low cadence work. Also what bike are you planning on putting on it? If it’s not 700c with road gearing then it may also be better to go variable resistance as a most turbos are tuned to work best with a road bike. With a budget of 150 quid you’ll probably get a better mag than a fluid one.
Will it be a permanent setup? If not then it’s worth getting one that is easier to get the bike in and out of (i.e. has a cam to release the roller instead of an adjusting wheel.)
If you are serious about training and think you may be doing a lot of turbo work then it’s worth having a look at TrainerRoad.com and making sure you get a turbo that it knows about (there’s a bit at the bottom of this page to check if the trainer is supported.)
Also depending on what you are using the trainer for then it may be worth thinking about a set of rollers instead (these can be had for 150 quid.) Will help with balance and core work as well as the legs.
Lots to think about really. Though if you just want to do a bit of spinning over the winter then the Elite one mentioned above, or one of the cheaper ones on Planet X would do the job just fine. (I won’t do the usual STW thing and recommend what I have as it’s a bit out of budget!)
Also it’s worth budgeting for a big fan (I got this one from screwfix.) A turbo specific rear tyre is also worth having as it won’t make a mess of the turbo in the same way that a road tyre will.