From what I can gather (dyno's and such), either one will gain you 5 hp. The airbox offers all of the restriction on our motors. If you want a chrome tube and emissions legality, get the Mac. (And you would be supporting Explorer Express which is always a good thing) Or if you want the same performance for less money and don't have emissions testing, then get the KKM. The intake tube for the SOHC doesn't offer much resistance, even KKM said that upgrading from the stock tube won't give you a noticeable gain on the SOHC (in the KKM intake tube thread) but people want em so he'll make em.
But we're only talking a $40 difference, so that may be worth the piece of mind emissions-wise and knowing you're supporting John V.
Just my $0.02
I personally would've gotten the Mac if it were available when I was buying my KKM, but thats just me.
