Hi there Gpaint!
Honest opinions.. That's my thing!
So there is a direct correlation between size of the rad and performance.
Furthermore a larger 120mm fan will quite often be able to move more air than a 92mm.
Now, I've heard of people using the 645LT in a system like yours, with decent results. Especially if you can manage to cram something like a Noctua NF-A9x25 onto the radiator. If you are in it for optimal performance both thermally and acoustically, I'd suggest getting a 120mm with a 25mm thick fan on it.
I hope this provides some clarification - otherwise I'm always happy to help