I honestly believe that the red UFC will look and fit much better in the cockpit than the newer LCD green one in my opinion.
First, I think it stands out from the MFDs. The main reason why I like the F-15E cockpit so much is the perfect blend of gauges and modern displays. Only digital stuff and not many gauges and switches such as the F-35 or maybe the F-22 a little makes it look quite dull... and the red UFC makes a big difference in the F-15E. The green LCD UFC gets rid of much of the nice blend the original cockpit had when looking straight forward. It does make the front view look a bit dull, we will have too much green luminescence and not much else to give it a nice contrast, something the original cockpit has and in a very nice way.
Second, the newer one looks kinda out of place for some reason, maybe the the emptiness of the edge on top of it give me a strange impression... it's ugly, at least for me.
Third, hmmm, well... the red LEDs and the overall 90s calculator style is just too cool!
Well that's just what I think :) In the end I'm sure the final product will look great anyway. :)