I am making the same decision you are, between the 133A and the upcoming Via DDR chipset. I have come to the conclusion that from a price/performance standpoint, the 133A is the way to go. With current processors, the performance gain is from roughly 5-15% depending on application using DDR. This rather small performance increase will come at a cost. The DDR Via boards will be somewhat more expensive, as will DDR memory. SDRAM prices are insanely low (I saw at Crucial ~$96 for CAS2 133 256 MB!!). And, I came to the conclusion that money I would have spent on the more expensive DDR setup components, I will put towards a faster processor, and a RAID array (probably will come out to a bit more, but the performance increase will be more pronounced).
Also, the KT133A is a very mature chipset (really, it is the original Via Athlon chipset with Socket A, and 133 FSB support) and the current crop of boards are rock solid stable with features galore.
That is my take on it anyway.