Why would I choose a bike trainer over a stationary bike?
The main difference is cost, both money and space.  If you already own a bicycle then it is using up space whether you can ride it or not.
If you want to use the existing bike on a trainer then that may end up cheaper overall.  And you can still unhook the trainer and go ride normally, giving you dual-service.
By comparison, a stationary bike is a fixed item and can't be used to ride outside.  Its a single-purpose tool, and takes just as much space as your normal bike.
Other points to consider

Noise - I'm not sure which is going to be louder, and this may not be an issue in your environment.
Wear on bike - adding mileage to your real bike is hard to track when its not moving.  For a stationary bike, its total distance travelled is unimportant.
Training effect - I can't really speak to this, but there's a significant difference between any indoor bike and riding for real.  The &quot;crank inertial load&quot; drops off much quicker on any indoor bike compared to an outdoor ride.  That is, you stop pedalling and coast much longer for real.  Expensive trainers can duplicate that better, but its still not ideal.
BORING I find any static bike to be mind-numbingly boring after a while.    But I've never bought into the virtual options like zwift, so that is an additional cost to consider.   Turning the pedals over while reading a book or watching a movie feels quite pointless to me, you're not concentrating on the ride.

Ultimately its up to you - Do you want a bike that you can ride for real when conditions improve?  Or do you want a piece of inside exercise equipment that risks being left unused after a while?
At least the bike can be used outside.  The stationary bike means you still need to buy a bike for normal outside usage.

I've just realised the suggested bike is a hardtail MTB, and presume that you intend to ride off-road with it.  So there's a sub-question here about

whether the kind of bike you intend on riding, and how/where you want to ride affects the choice between stationary bike vs a bike on rollers/trainer.

I can't give a definitive answer on this point; anyone with experience is welcome to add their own answer.