The biggest advantage to an observatory IMHO is that the scope can be left set up and polar aligned. Saves a heap of time!
I have always preferred a roll off roof type obs, but you may have some fun with council getting a roll off roof approved. I've never been a fan of a modified garden shed since most garden sheds are light/flimsy to say the least so you have to put a lot of work into them to make them a) secure and b) robust enough to withstand the worst storm you can expect to protect your scope.
When I enquired at council about the POD and showed them the pictures and drawings from the web and they said it was an exempt development - I was sold!
Since having the POD it has excellent wind protection away from the direction of the wind with (if you set your scope up properly) a view to the horizon - not that I've used the view to the horizon yet
. Its probably only comets that I'd chase so close to the ground.
Al.
Al.