Im stuck. The intel i5 processor on the apple iMac is a quad core with 2.66GHz but the intel i3 on apple imac has 3.06 GHz. I would use this computer for video making, web use and downloading large files. Even if its a tiny difference which one is faster? which one would make your computer feel and seem faster?

Since you make videos, the i5 quad core will be more beneficial because video editing/making makes full use of cores. For videos the i5 will be about 1.8x as fast as the i3 which you mention. And you probably won't notice the slightest bit of a slowdown doing everyday tasks with the 2.66GHz vs the 3.06

Get the i5 and you will not regret it.