Greenhouse vs. Polytunnel – Which One Is Better?

What is the difference between these two things? Isn't there also a plastic greenhouse？

I refer to them both as high tunnels.  Looking at the picture, one appears to be open ended.

They are actually two different kind of greenhouses.

A poly tunnel uses polyethylene, which is a floppy plastic 6 mils to 12 mils thick.  It's often labeled greenhouse plastic "film" and it lasts longer than a roll of clear plastic (that isn't labeled polyethylene) that doesn't have any sun protection built in.  Rolls of polyethylene come in 10-feet, 12-feet, wide widths to cover hoops and frames.  There's special clear tape for polyethylene that holds it to the frames, can endure the weather, but is quite expensive.  It's easy to wrap the edges of this floppy plastic around a 1" by 1" or 2" by 1" trim-sized boards, then screw or nail those boards onto a PVC or wooden frame.  This kind of film needs replacing every 5-8 years, assuming a storm doesn't cause damage or the environment isn't too harsh.  Mice and packrats can easily eat through

A plastic greenhouse refers to polycarbonate panels that are stiff from end to end, but will flex from side to side over a hoop structure.  Check for how much light they let through, because some are 80% and some ae 90%.  You want the ones with the most light.  It's the protection they put on the panels that blocks out the sun a little bit, but I've never had it be a problem.  The panels you can get at a home improvement store are usually 2 feet by 10 feet, they overlap a bit, and they need to be fastened down on all the edges of each piece because the wind can catch them and flip them up in the air.   The higher priced ones can actually withstand being lifted and bent by a storm wind, so it's better to spend a little more money and get the good ones.  Polycarbonate panels can last 2-3 times longer than polyethylene film.  

In a windy area a hoop house is a good shape because the wind slides over it better than a square frame with a peaked roof.  It's also easier to replace because the film or the panels easily slide over the gently rounded shape.  There's no roof to climb on.  But maybe the shape of a greenhouse involves preference.

I have both kinds.  The polycarbonate panels last a really long time.  The other polyethylene film becomes a landfill product sooner.