I started with a 6x8 glass green house which was great,easy to maintain temperature in but was too small to produce what I wanted to produce,a polytunnel is used in Scotland as a means of extending the season a bit and ripening stuff that fail outside in all but the best of summers,(sweetcorn is a prime example) and of course allows you to grow a lot more varieties of suitable crops and save a bit of money by growing bedding and vegetables for outside from seed.If you can both afford and have the room then I guess both is the perfect scenario.