Er, maybe neither?
I agree with Phantom Piggy that EQ should be applied to already recorded tracks, not while tracking.
Pre amp? Frankly, the differences made by most mic pre amps are pretty subtle--and the effects typically applied in your amp or with various foot pedals are anything but subtle. I doubt a pre amp change would even be noticeable.
I know that Gear Acquisition Syndrome can have a powerful effect but, instead of coming up with two very different bits are hardware and asking which to buy, you should have a specific idea about what you want to change in your recordings and then decide what you need to buy to achieve this change. It could be anything from a new totally different microphone to better monitors to acoustic treatment in your studio.