I have both but one is inside the other - obviously the greenhouse inside nthe tunnel. Ours is only 6'x8' and takes up about half of one bay of the 7x14metre tunnel (sorry about mixing metric and imperial....) I use the greenhouse for bringing on seedlings once they have outgrown the propagator in the bedroom, then I use it for chillies, peppers, aubergines and tender things like that. We are at 1000', windy and cold, so a greenhouse outside here would just explode, or implode or otherwise disintegrate in the first big wind.
However, it does take up more space than it warrants and I think that I could manage with cold frames in the tunnel, if I didn't already have the greenhouse. I have tried using the greenhouse to overwinter tender plants but the temp in the tunnel is about 1 degree higher than outside, and inside the greenhouse about another degree higher, at night, which means that it's still way below zero, whereas in the day, in the summer, the temp can soar to well over 100F.
Whether or not you sell the greenhouse could depend on how much you would get for it, how big your tunnel will be and if you grow lots of plants which need a greenhouse rather than a tunnel. I am a hoarder so would never get rid of it