What would be Faster?

I'm ironing out some details as far as how to BTO a 27" iMac. I'm currently trying to decide between the 2 and 3tb fusion drives. and wondering if $100 difference would be better spent elsewhere. The machine would be mainly used for imaging in PS and LR. I need to know what would be faster as far as read and write, Apples fusion where the HDD runs @ 5400rpm or an external USB 3 drive running @ 5900rpm that I currently own. Internally, is it based on bus speed?

I suspect the fusion would be faster, but want to make sure. If so, I'll order the 3tb for now, wait a few months, maybe more, then go with a thunderbolt enclosure with HDD's at 7200.

Opinions? Suggestions?

Thx