I'm going to buy a 10mm upper and was wondering what buffer should I run. I'm thinking the 9mm heavy buffer and standard spring.

If you is using a "CAR" length tube/stock, then a  5.5oz  "9mm" CAR buffer should be fine, along with a CAR length spring.  If you want more weight, look at the  "A", "B",  or "X" buffers from Ken Elmore,  or the  "Q" or "45" by Slash....


If you have the rifle length tube/stock,  then the std rifle buffer and spring will suffice.  If you decide you want more weight, you can dump the guts, and install a machined brass dowel, that should get you to 7oz,  or make a custom steel buffer and trim it to the desired weight.