Hello there,
When I was looking for a new MTB as my old Giant wasn't fit for the job- fancied something newer and better. I went to a local bike shop and asked them what they recommended, I told them that I had a budget of £350 (similar to yours) and they pointed me in the direction of Marins and Giants.
Aways I asked them If I could test try the best looking bikes (for riding not in actual looks) and I left a deposit and my Dad in the store and went up and down the road on the Marin and then a Giant, sprinting, low gear, brakes, saddle height, if the length felt right. Being me- I wanted another go on the Marin, so I did... Then I'd realised It would be a close call between the two, so I looked at the components. The Marin had a free disk brake upgrade and a lifetime warrenty- the Giant had vee's and 1 year's warrenty. I went for the Marin.
While cycling the south downs way (hilly MTB route) home 10 miles, on flat platform pedals. I'd realise that I wouldn't like the Marin's riding style straight away so halfway through the ride going up some hills I normally had no problem with- I was reduced to walking pace... Felt like I'd got a shit bike- also got TWO punctures in the same ride home...
But looking back at that now, I realise I'd suffered the price to pay for buying an awesome bike that will last me a long while!
So my reccomendations to you are, go to a bike shop, go with an adult/parent you know that know a decent amount about bikes, tell them your budget and get them to point out their recommended bikes at the price, then ask if you can test try (no doubtedly they'll say yes as they want you as a buyer!). If your unsure about the bikes, go home, come back another time or go to another shop and just see which one feels right.
Sorry about the super-long post but I hope that helped!