Both queries are now fully optimized and that lead to different plans. First of all, both queries run in a Batch Mode, which is much faster than a Row Mode.
In the first query, we see Hash Match Aggregate instead of Stream Aggregate, more to the point you may see that Actual Number of Rows is 0, because all the rows were aggregated locally at the Storage Engine level, you may see property Actual Number of Locally Aggregated Rows = 60855. This is faster than a regular aggregation and is known as Aggregate Pushdown.
In the second query, you may observe a new Window Aggregate operator which is faster than a Window Spool and runs in Batch Mode also.
Read the whole thing. Dmitry also looks at SQL Server vNext and how it handles the same trivial-plan-generating scenario.