How (and how much) are you planning to ride?
Will you ride hard enough to get sweaty?
Are you gonna ride standing regularly to cope with lumps and bumps on the track?
If yes, you're not going to get a reliable bike new from shop for $300.
Even at $500 its pretty much borderline.
Department store MTBs aren't MTBs, they're only dressed to look the part. If you check the fine print in the warranty it'll say "for casual use only."
It'll also say "warranty only valid If assembled by a professional".
If you buy boxed and assemble yourself, you lose what little warranty you ever had.
Department store bikes are put together from the cheapest parts by the lowest paid. Sloppy and inaccurate work is the norm.
If you still decide on that route, agree with a real shop beforehand to assemble and adjust the bikes. Bearing adjustment, spoke tension, brake and derailer tuning.
The bike industry is very homogenous.
Ignoring sales and special offers, Comparably priced bikes will have comparable quality and performance regardless of what logo the frame carries.
If you're not looking to get sweaty, forget about MTBs. Go for hybrids.
They'll hold up just fine and be generally more useful to you than MTBs.
But again, not for $300.
How much difference you'll notice is difficult to say.
Pootling around on flat ground, staying in the same gear - not much.
Trying a late shift during a climb, braking hard, doing a standing climb, popping a little wheelie to get over/onto a widewalk - then you'll notice considerable difference.
Buying used can offer great value for money - If you know what you're buying.
Sized wrong, all beat up can quickly turn the purchase sour.
If you can find them, shops selling trade-ins, returns or bikes never retrieved after repair can be a good middle way. Shop bikes at department prices.
Or If you have a friend who knows bikes and can help you pick used bikes for you.