Re: 300 mag vs 7mm mag sendero
I shoot a 7mm and love it. 168 Bergers at 3075 gps is very impressive. The 7 mm has as many good bullets as the 30cal. But the 300 win mag is equally impressive.
As for a muzzle break, is it absolutely need, No! Does it mke it much nicer to shoot and spot your shots, Yes. I would put a break on it.
Both will suite you great.