Which one is faster depends on what you are doing. Matched RAM I would guess at 6 - 8% improvement. (35%-50% is the territory of XBench synthetic benchmarks, which are only one step removed from outright fiction)
But here's the thing... as soon as your programs and data consume more than the 2 Gb of physical RAM and you start paging to the hard drive, the memory performance plummets. At that point, having the extra 1 Gb RAM in the 3 Gb config is way faster. Until you open a bunch more programs and break the 3 Gb barrier that is, and then it all goes in a handbasket anyway.
So if you are doing digital audio, video, big Photoshop, etc. with programs and data sets that can use more than 2 Gb, then the unmatched 3 Gb is your ticket.
Thanks
Trevor
CanadaRAM.com