This past season was my first time going to both Castle Mountain and Fernie. I personally prefer castle because it has a good park if you get bad conditions and I find the terrain much more fun. If you get a good snow day it will all be skied out by 10:30 although the first hour and a half is always epic. If you don't get any snow Fernie can get pretty boring from what I have skied there. Its all pretty mellow, no steep lines or good drops (besides one small 10 footer right by the timber chair.) If you can find someone who knows the mountain well there are some really good places to hike to but that takes a lot of time. Also if you're skiing there for more than 2 days you will probably end up wanting to kill yourself because the mountain is swarming with untold amounts of gapers and weekend warriors who think the only usable ski on the mountain are Obsetheds and Bent Chetlers. If you do manage to find a good group of people to ski with then it should be fine, you'll probably end up ripping on half the people on the mountain which is fun.
Castle on the other hand has fewer big snow dumps that I am aware of but has way more stashes you can find all season long. I was up there for a competition and managed to find a run that was almost completely untouched and was a joy to ride over and over. It also has a good amount of terrain that you can hike to within ten minutes along with some stuff farther away. Its a super fun mountain to shred with friends because it has a lot of natural airs everywhere along with some signs that are wide open for tapping. I think it would really fun on a pow day. They also have a good park, none of the jumps are very big but they are well built and have decent sized gaps. The rails and jibs there are also very fun.
I'm pretty sure there are people here who know more about Fernie than I do however my experience of skiing there for a three day weekend was pretty bad even though one of the nights it snowed 30cm. You will probably have fun at either hill, I just think castle is a better mountain.