Typically I've found that DISTINCT is faster than GROUP BY when SELECTing a single column, i.e. as below, but obviously that could vary:
SELECT DISTINCT
SSF.customerId
FROM #example SSF
WHERE SSF.sales IS NOT NULL
--vs.
SELECT
SSF.customerId
FROM #example SSF
WHERE SSF.sales IS NOT NULL
GROUP BY SSF.customerId
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".