I agree with what others have said regarding the 7mm ep. I've found that's the most used one for me on planets.
With my 10" dob @ 1200 F/L it gives me 170 X magnification.
But there's still plenty of times I've been able to use the 5mm, despite living amongst light pollution in the suburbs. It's had enough use that I justified getting the 3.5mm- some nights have been pretty amazing for planets and I wished I could've gone lower. Granted, it won't be used a lot, but it's there when needed
. And it should blow me away when observing the Homunculus nebula in eta Carina and splitting tight double stars
.