Were you thinking of the Sigma 80-400mm f/4.5-5.6 EX DG APO OS? I'll be potentially very irritating now and say "neither" if you have any chance of affording the EF 100-400mm f/4.5-5.6L IS USM
, also reviewed very positively by PhotoZone here
. The only drawbacks with this lens, cost aside, that I can see are a push me/pull you zoom and an early generation IS. No signs, yet, that Canon are introducing a MkII so it should hold its value better than either of the third party offerings. Canon currently have a cash back offer on the lens here in Europe so maybe in a month or two the USA will follow suit.