Hugo Kornelis (7/14/2010)
Carlo, you are missing an impportant point in your repro.
The performance difference in the question is not caused by rebuilding or converting the table contents, but by rebuilding / converting the nonclustered index. Since your repro lacks a nonclustered index, it's not a surprise that you don't see a performance difference.
OK, for performance question.
But the two options do very different things: option 2 changes the column from NOT NULLABLE TO NULLABLE.
The hint is to enlarge a column not to change attribute.