Before a magmount, I would say it's always best to use a "real" mount.
#1 drilling a hole in the roof and mounting a NMO
#2 drilling a hole in the trunk and mounting a NMO
#3 using a "L" bracket on the trunk (if on a car)
#4 a trunklip mount (if on a car)
#5 a magmount
If you still have to use a magmount for whatever reasons, go with as high quality as you can afford. Also, how well can you RX the system now? If you pick up everything you can now, why do you want an external antenna? Sometimes bigger is not better. I have one system that I usually take off the RS800 antenna and put up a stubby to knock out some of the nextel and other interference.