With Sierra-2 speakers on the left and right, I can't say for certain whether you'll be better off with another Sierra-2 or a CMT-340 as a center speaker. But I'd guess that you'll be happier with another Sierra-2.
You can save money on the rear channel speakers, such as with the NHT SuperZeros, but spend it on the front three speakers.
Does Ascend offer a trade up? First try the CMT-340 and if you want, later trade it in towards a Sierra-2.