About the only things that SiteMeter has that StatCounter doesn't are outclicks (and the folks at StatCounter are beta-testing that) and the general support of advertisers and TTLB. On the flip side, you do get detailed logs of the previous 500 page loads (versus 100 visitors), the option of an invisible counter or text-only counter (versus no option for either) and the full IP addresses (versus the last octet being obscured).
Could I use both SiteMeter and StatCounter?