Jason, I don't think we can call your test apples-to-apples, at least not "real-world" anyway. At least 95% of the real-world running totals cases I have ever seen are a) partitioned by one or more columns b) do not have any index that would help either method and c) are output queries, not updating a column on a table. Also IIRC there are something like 6 rules that MUST be adhered to when doing the Quirky Update and I think you only have one of them (the clustered index).

Whenever you get around to your next test can you construct it along those lines please? No worries if you are like me and too busy to get it set up! 🙂