A lot. It changes your speed, manoeuvrability, weapon load out.
Each one feels different, you'll find one you like while playing around.

By just doing the campaign you'll have enough to unlock all 3 final jets (F-22, F-35 and SU-57), but only if you beeline to them, avoiding buying other planes that aren't in the way. If you grab some earlier jets along the way, you'll still have enough to buy at least one of them before the end of campaign. And you can always just farm in free missions, so it's not like you'll be locked out of any jet

Carrier based jets are mostly needed in the first half or even third of the game. It's also a lot easier when you know what tools are required at certain points of the campaign. Not having them doesn't prevent you from progressing though.

Try to get the F35 as soon as you can. You can use it on ground and carrier takeoffs, and it's also multirole. Comparatively. it's not as good as the F22 or Su57, but it's flexibility more than makes up for it.

Iam sorry but they are definetely not the same the more expensive Planes have way higher stats you cant compare an f 14a to a su35 for example or you telling me the frogfoot has mostly the same stats then an 550k plane?