I use the 30ft meter on 5-8ft putts. I subscribe to the ram it in philosophy rather than falling in the door drunk.
I like the 10 footer for 3-4ft putts, or longer putts with severe downhill. I like to know exactly where 4 feet is rather than guess where it is with the 30 footer (I'm old school, I don't use ShotPal or PutterPal and never will).
Plus, and this is probably just perception, but it seems like the ball comes off a lot softer on a 10ft meter versus the 15ft. But like I said, that's probably just me wanting to believe that.