Do You Want an Under- or Above-Ground Shelter?
To me, this one's a no-brainer. You want your bug out shelter to be underground for the following reasons:
Security. Planes flying over and people walking through the woods will be able to spot a normal home or shelter. If they see smoke coming from the chimney, they'll know it's inhabited. This could invite unwelcome attention. An underground shelter is largely protected from those who may try to smoke you out of a normal structure by lighting it on fire.
Protection from the weather. An underground shelter is protected from all sorts of inclement weather. The only kind of weather it isn't ideal for is heavy snow, because you may end up having to dig yourself out of ten to twenty feet of snow if you need to leave.
Warm in the winter and cool in the summer. Underground bunkers aren't impervious to hot or cold days, but they're a lot easier to cool off or heat up.
You can hide for a long time. Going underground protects you from the bad stuff going on up on the surface. Just make sure you have a source of filtered air or you run the risk of letting poisoned or radiated air into your bunker.
You can get as elaborate as you want to with your underground bunker. I've seen high-tech bunkers capable of filtering out poisonous air. I've also seen what amounts to a couple of reinforced shipping containers buried underground with PVC pipes extending to the surface for air.
No matter what you decide on, you're going to want to give yourself an alternate escape route. If you're found out and there are intruders beating down your front door, an alternate escape route will allow you to get the drop on the people trying to get in—or it'll allow you time to slink off into the woods, your choice.