All depends what you want it for. If you grow all your own stuff, a greenhouse, but if you just pot up bulbs and re-pot larger plants that are to go into the garden then a potting shed. Although the potting shed has a glass side to it, i would of thought seedlings would tend to grow slightly towards the light and end up etiolated (stretched) as they would have to grow more to reach light.
Plus a greenhouse allows you to be able to grow tomatoes, cucumbers etc. during the summer months and also seedlings and cuttings

do you want the best type or the benefits of both' you can bie a green house potting shed combined, which if you want to potter in so to speak they are ideal, but if you want to do some searious growing, like selling tomatoes, or you want to grow a number of things then spend on a as big as you can afford greenhouse, making shore that you can open as much as posable on them,

I absolutely love my greenhouse - I sometimes just sit and read in there - like having a conservatory lol - but with compost LOL

I,d love a big greenhouse but where I live it's not possible, it would be like having a large sign saying " throw bricks here" .lol.
I have to do with my mini greenhouses and coldframes and have my shed which I sometimes sit in and read my gardening books as well as potting stuff up.

A tidy one!!