If you want, you can direct drive a P7, like off a single lithium ion cell. One battery, no driver necessary.
To power an MC-E, you don't have that simple direct drive option. You have to "juggle" the combination of: how you want to wire the MC-E (4p, 2s2p, 4s), whether you want a buck or boost circuit since you have to use some sort of driver, and decide what battery combination to use to not only give you the runtime you want, but also the necessary voltage.
I'm not saying that this situation is any better or worse, but it's a difference between the P7 and MC-E.
If you wire a bare MC-E emitter in 4P, it is electrically the same, for all intents and purposes, as a P7. And as others have stated, you can also buy an MC-E on a star in 4P if you don't want to do any wiring.
And whether the LED is a P7 or a (4S, 2S2P, 4P) MC-E, you still have to "juggle" driver selection except if you decide to go direct drive.
Some actual differences that might matter include:
- Different die heights
- Different dome dimensions
- Different case dimensions and shape
- P7 slug is tied to the anode, MC-E slug is not
- P7 dome is the gummy dome, MC-E dome is not
- P7 is selectable for Vf bin, MC-E is not
- MC-E 4P and 2S2P options provide flexibility for replacing various different LEDs (e.g., Seoul P4, Luxeon V)
- MC-E 4S option avoids any issue of die mismatch that can lead to thermal runaway