Well you have to be a good shot first to know the difference lol
Like, your going to learn how to drive soon. Drive a brand new dodge ram 1500, then go drive a base model hyundai accent, then go drive a volkswagen jetta highline.
Your going to think they all drive really really nice, but for someone who's been driving for 20 years, they all drive way differently.
Anyway, quality is really important. Especially when it comes to upgrading. Most cheaper guns are only 70% compatible with upgrade parts 50% of the time.
And the other really huge benefit to quality is durability. My G&P 249 is worth $600 more than a classic army 249. Because it's made of better steel, you can run trip and fall flat on the BOX MAG and it will not break.
You look at a STAR M249 the wrong way and it's front sight breaks off lol