Things to consider:
What do you want to grow? “Crops” may be easy in a tunnel, a collection of pot plants for the house may be easier in a glass house.
The smaller the house the more difficult it is to get an even growing environment, and to keep Summer temperatures down, IMHO.
Glass with top vents and a louvre down low offers a nice atmosphere – air is drawn in though the louvre, the heat rises and exits through the vents, so here is very little “draught” – draught causes the plants to transpire and thus dry out more quickly, which in turn means they are more stressed, and perform less well. With Tunnels you open the ends, and the breeze blows through (but if you point the end at the prevailing wind then a gale may make it take off!). Glass normally has vertical sides, so easier to grow up to the edge, or to have a table/staging up to the edge, although some Tunnels are shaped with near-vertical sides.
Tunnel is cheaper (although check eBay for second hand greenhouses, bargains are available) but needs the plastic replacing every few years.
Glass houses are prone to yobs throwing stones (Polycarbonate or toughened-glass [expensive] are possible solutions to that threat) and may be a hazard to small children – don’t site them at the bottom of the hill the kids race their bikes down!
Tips for Constructing a Greenhouse
Consider where you will site a water butt to collect the rain from the gutters – the back means that you can join both gutters to a single water butt (no door in the way!) the front means you have less far to walk to fill the watering can! It helps to very gently slope the foundations / base towards the water butt end.
One thing I wish I had done is to put a duct (100mm training pipe or similar) under / through the foundations so that I could get a mains water pipe into the greenhouse, and possible even electricity in the future.
I you have borders, rather than solid floor, consider digging it before erecting the greenhouse, or at least before glazing it. I have heavy clay, and it set like concrete once the dry glass cover was up!
I have lowered the soil in my greenhouse to about 4″ to 6″ below the foundations (and there are then two courses of bricks making up the base), which gives me a bit of extra cropping height to the eaves.
The higher you make the base the more cropping height you have, but the bigger the lintel that you have to step over to get in! I wanted to be able to get a wheel barrow into my greenhouse, so I created a ramp on the outside, and inside, to get up to the height of the door. Otherwise you may need a step.
Don’t screw the frame down until the glass is in. You will be straightening the frame as you put the glass in, so may want some wiggle-room – that is provided you can get to the anchoring holes after the glass is in – if not perhaps glaze the roof first, then anchor, then glaze the sides.
Plans for a DIY Tunnel
“If doing a brick base (i.e. not using a metal base) please use a solid brick with no frogs or holes. Give yourself enough room around your base to allow you to anchor the base into the ground.
OVERLAPPING CILS DEMONSTRATES THE NEED FOR ACCURACY WHEN BUILDING BRICK BASES”
“Before you begin building the roof you should firstly check that the internal diagonal measurements of your greenhouse are equal in length. This will indicate that the frame is square. The frame also needs to be level in every direction so the glass can fit into the frame squarely and your door runs smoothly.”
I have written an article about my two greenhouse, which I bought second hand on eBay