MS if you need runtime. P7 otherwise. IMO.

I have both. I bought the MS because I was used to 200 lumen lights, and need 50 to 60 minutes runtime in the winter. I saw "45 minute runtime" on the P7 and said "not enough" and bought a MS.

Turns out, the P7 is so bright that I only need to run it on medium and it's still more light than the Dinotte 200L that I was used to; plenty to ride by in pitch black in the winter, except on the worst parts of my ride where I kick it to high for about 10 minutes. All told I got plenty of runtime to finish my ride, even in the winter when the ride is longest and the batteries are coldest.

The MS is a good light too, but there's a lot more to go wrong with it; wires fray, connectors get screwed up, the battery isn't waterproof, you have to route wires, take the battery on and off to charge (not good to charge in very cold temps in the winter, so take it inside).

A flashlight on a bar mount is as simple as it gets, it's waterproof, and for only a few bucks more than the MS you can buy two of them and have a backup (batteries and chargers don't have to be bought twice). IMO you should have a backup anyway; ANY light can fail even if you spend $1000 on it.

Also, you can grab a flashlight off the bar and use it as a flashlight. Very handy. Or repurpose it easily as a helmet light with just a few rubber bands; perhaps even just to change a tire by the side of the road at night.