CTSV vs STSV
If you are comparing the 07 CTS-v with the 07 STS-v then the STS-v wins handsdown. During the early naughts (2001-2005) Cadillac was making rapid progress in dynamically upgrading their vehicles - the structural underpinnings; the design, the fit and finish and the interiors. So there is a lot of difference between the 03 CTS and the 05 STS. The CTS came out before the Europe-born Bob Lutz had the opportunity to rally the troops and demand nothing less than the best designs and interior standards from his troops. It remains to be seen, now that he is gone, whether Cadillac will continue to reassert itself as the 'Standard of the World' or just go back to being a provincial US brand with very little appeal to global drivers in the same way that Lincoln has given up the ghost. I am not optimistic now that Government Motors is being run from DC and not Motown.
I own an 08 STS-v but I keep it garaged in Rome. It is a phenomenal vehicle and it really shines on twisty European highways at between 100-150 miles per hour. Once on the Civitavecchia-Rome highway I raced a German-plated Mercedes 500SL for approximately 40 kilometers. Each time he tried to pull away I moved up his rear bumper and flashed him to move over. When we slowed down for the tolls he flanked me and I could see from his expression he was completely confused. He must have been asking himself how a Cadillac could have bested him in top speed and acceleration. If you live Stateside I am not sure you will be able to push the speed envelope to the same extent we can in Europe where we can drive these cars at top speed for hours in Italy, Germany and Spain without worrying about distracted meandering drivers and surly cops with no sense of humor.
However, if you are asking me whether you should buy an 07 STSv or an 09 CTSv then buy the new CTSv. Like I said previously GM has made major progress in its Cadillac line until now. That may stop as we speak and the 09 CTSv will remain the apogee of a bygone era. If you want me to elaborate on the dynamic qualities of the STS-v then I can contact you directly to discuss. Let me know.