Imac mac pro what to do

[Enny5]

Hi guys i am about to purchase my first mac. I am editor by profession and graphic/motion designer using programs such as cinema 4d real flow and after effects. Right now i can afford Imac i7 2.8 ghz 27 inch but i can also afford mac pro 8 core 2.2 ghz that is refurbished but i am wondering if Imac is faster when it comes to rendering since it is 2.8ghz but it only has 4 cores. And if not what would be the rendering difference in time? Another thing is i will be using this mac mainly for learning Apple final cut studio but my concern is will apple color and motion run on Imac i7. And what about external HD can FCP on i mac edit HD footage or pro rez? And ow long can Imac be use without purchasing new workstation.