Question

So, is Jolly different from other "speed" natures or do they have the same speeds? Please help! Thanks in advance!

Answer 1

No, They don not increases differently. A nature increases a specific stat with 10%, and reduce a specific stat with 10%. Jolly Increases speed stat, and reduce special attack (+Spd, -SAtk).
So, yes it's different from other "Speed" natures, e.g. Timid nature increases Speed stat, and reduce attack (+Spd, -Atk).

Answer 2

No. All of the natures will be the same regardless of which one you boost. You don't get a higher speed boost for a specific one, so it makes no difference.

The only difference would be something like a serious nature, since it boosts and lowers speed by the same amount, so it makes no difference.