LTX 2.5 Pro vs LTX 2.5 Fast: which one to use

The shot decides this one before you do. LTX 2.5 Fast renders up to 4K and runs as long as 20 seconds. LTX 2.5 Pro stops at 1080p and 10 seconds, and puts the difference into per-frame quality instead. That is the part the tier names do not tell you.