[tor-relays] "NumCPUs 2" or 2 copies of Tor?

[Steve Snyder]

Given plenty of RAM, a muti-core CPU and a single IP address, which is 
more efficient:

a single instance of Tor configured with "NumCPUs 2" or 2 instances of 
Tor configured with "NumCPUs 1" and different port numbers?

Thanks.