POTS si first. Then VOIP...then the best match. So if you have a POTS and VOIP peer that match the POTS will be used first. Then the most specific will get used.
POTS-
destination pattern 12..
VOIP-
destination pattern 12..
POTS will get used if you dial 1234
POTS-
destination pattern 12..
VOIP-
destination pattern 123.
VOIP will get used if you dial 1234.
The preference command wont work between voip and pots. Only pots to pots and voip to voip.
To determine what peer will get used, use "show dialplan number #"