Hi susie1
I am no expert but I will share my experience I built my poly tunnel 8 * 20
I have a windy site so used half scaffold pipes driven into the ground then used thick blue water piping for the roof support but this withstands high winds no problems.
I have a central slab path down the middle.
I have doors either end with a mesh cover to allow air circulation and open the door for periods to allow insects in for pollination.
I do not have experience of the greenhouse but I love my poly tunnel.
I plant successfully chillis cucumbers tomatoes winter salads tomatillos.
One mistake I made was installing overhead watering system the watched in horror as blight developed so I now usage ground level watering system.
Finally I spent more on the polythene cover to give good light throughput and uv it has so far lasted very well
So I am in favour of the poly tunnel
Regards