Polytunnel or Greenhouse?

[frostyfingers]

This year I've used a poxy plastic 6x4 greenhouse which was great for planting and bringing on seeds and has been good for cucumbers, however my tomatoes are not happy and I think it's to do with the ventilation and heat (doesn't get very hot, and gets very misty even with window and door open).
And it's too small.

So, we've decided to go large next year, but would like to hear the pros and cons of either a greenhouse or polytunnel. We have space for probably a 8x10 of either, the ground is unlevel and would need digging out for a greenhouse, so a polytunnel would probably be easier in that respect.

But, do they ventilate well, what's the best way of ventilating and what is their life expectancy....

Any wise ones out there who can point me in the right direction please?

Well we have both but use them for different things.
The greenhouse is good for winter and early spring where we can give gentle heat and they get loads of light .Sowing seeds and growing them on and for over wintering things.But later in the year it just seems too hot whatever we do and we've tried various types of shading.
polytunnels are fantastic for later on until harvest.we bought the frame on ebay and then the plastic separately .We bulit a wooden framework with door for either end and a 'pea-net curtain so that we can leave the doors open to get a breeze through, but stop birds and caterpillars from getting in.
If I could only go for one it would be the polytunnel

I would love a polytunnel. Think they might be less of an issue in windy areas.