I am certainly no expert, but I think vehicle weight & engine power are critical for this decision. Assuming 900+ hp and under 3000 lbs for a track only car that runs on well prepped surface the powerglide will be less expensive to buy, less expensive to maintain, and slightly faster with the right converter. If the car is heavy, less power, or needs 2nd gear launch for no prep then a 727 or TH400 may be better.