Hardwood would be better IMHO .... as it would last longer. Cedar piles seem to last 3yrs tops around here, and they're not real good fish holding spots until the year after put in.

A year around fishing spot can be tricky if the lake is drawn down very much during the Winter. 15ft deep is the norm for most of the brushpiles put out by the KDFWR ... but even they've gone to using pallet stacks, PVC, and various other materials for many of their habitat projects. Now, they do use a lot of Cedar trees that they've collected during the Christmas season, and they do make some large piles with them .... but, those deteriorate pretty quickly and in some waters can get so gummed up with mosses that they're pretty useless thru the Summer to Fall stretch.

If the lake gets a thermocline .... you'll want some part of the tops of your piles to be above the depth it usually occurs, so that fish can suspend above it when the thermocline is present, and still key in on it and get down in it when there is no thermocline.