I'd vote power for many reasons. I think that you should expand your poll to include drum or disc too though. I'm running power 4 wheel drums now and I'd say compared to before, pedal effort is so much less it's amazing. I can't wait till I go to disc. I'll say that I also drove with manual drums for 1 1/2 years straight and I never had a problem. And now my left leg can leap over tall buildings in a single bound lol.
I put 60,000 miles on a car with manual brakes and I thought they were ok. That was a while back. Just about everything Iv'e driven since then, has been power. When I got my 65 GT (with the stock 4 piston caliper discs and manual single master cylinder) the first time I drove it, I thought the brakes were gone. It is much better with the power booster and dual MC kit I got from Mustang Depot. Now it is safer and "normal".
I drove over 100K miles on my old coupe and 50K miles on the Shelby with manual brakes without any complaints whatsoever. That being said I just got done with building a custom power brake setup on my fastback project so if I ever finish it I will have something to compare. So, I guess the jury is still out for me since I have never spent enough time behind the wheel of a power braked classic Mustang to know yet.
Originally I had six-cylinder 4 wheel non-power drum, that sucked. The car would barely stop, god help you if the drums got wet.
Now I have still have a non-powered setup although with the Granada swap in the front and 2.5in wide drums on the 9in. I don't care about converting to power. The car stops as fast as I want it to. There is greater pedal effort than a power brake car, but hey whatever. I would convert to power only if other people were to drive my car often (not going to happen).
I voted manual. I do think that it's unwise to run 4 wheel drums alongside modern cars in any kind of traffic. The disc/drum cars just stop so much faster that you need to leave a ton of room at all times. My leg used to get tired occasionally at long lights. Of course my wife and stepdaughters got around just fine driving the car as well, so I think it's a pretty minor thing.
Now that I have a disc/drum set up I believe power brakes are a luxury. I think that it's much easier to feel the brake response with manual brakes. Much better for a panic stop situation.
Also, if you want to run a high performance cam, the vacuum booster just might not do that well at idle.