I guess an M6 is great for the occasional blast. With a hotwired bulb, you are probably talking about no more than 20 minutes of runtime...a P7 can get an hour.
Will a P7 do a good job lighting up a field at night?
Well, that's not quite true. In stock form you're right, the MN21 will run for about 20 minutes. However, there is also the MN20 which will last for 1 hour, and the MN15 which will last for 2.5 hours. Don't write off the M6 and its hotwire (incandescants are called 'hotwire
s' not 'hotwire
d' because that's what they literally are) rechargeable bretheren...
The M6 puts out a really broad smooth cone of light with a lovely hotspot with a decent amount of throw, but note that due to the heavy stippling of the reflector, the M6 doesn't throw as well as a well collimated hotwire or to take it to extremes, an aspheric-ed LED.
So having said that, the P7s I've seen haven't been as bright and they don't have the wow factor of an M6 running a MN21. The P7 is very impressive in its own right though as a thrower or as a massive flooder but it doesn't just have the output to take on the 1000 bulb lumens of the MN21.
There are already comparison pics on CPF on this type of question - just do some hunting and you should turn them up. I think the best answer would be to buy both - both incan and LED are fantastic at what they do - but you need to try them both to figure out what suits your useage patterns best.