The interesting thing about the flagstone greenhouse is that it retains heat for much longer than the soil based greenhouse. The flagstones seem to act as storage heaters cooling down slowly during the night.
|Soil based greenhouse with mulch of chippings|
|Slab based greenhouse|
If your greenhouse is in full sun all day, allowing the slabs to heat up, then this is an advantage. My cold greenhouse (i.e. one without artificial heating) remains above freezing for most of the year. I know that we have had mild winters recently and this might not continue but it indicates to me that there is an advantage, particularly when you don't use any other heating. The slabs are on stone that was sieved from the rest of the allotment and this will add to the thermal mass of the slabs.
|Design behind the greenhouse.|
There also might be an advantage due to reflected light from the slabs particularly as they are grey/white. I think that this is a neglected feature that could benefit plants a lot. I keep the slabs as clean as I can to increase the light reflected and to reduce the possibility of disease. However, washing the glass and the poly carbonate sheets regularly, allowing for maximum light to pass through them, is also very advantageous.
Keeping the slabs clean with water has an additional benefit in the summer when the greenhouse needs cooling. Putting water on the slabs will cool the greenhouse through evaporation.
Further, my soil based greenhouse is slowly sinking even though it is on an aluminium foundation. I will have to remove the glass and polycarbonate from the greenhouse to straighten the foundations soon. Maybe during the winter. The slab one is not sinking.
Making holes in the slabs to screw the greenhouse down was very difficult, leading me to give up in the end. I am relying on the weight of the glass to stop it from toppling over in the wind. I have had it on slabs for over 30 years now (but not in the same place) and it has not even had a glass pane cracked by the wind. However, neither has the soil based one.
In the soil based greenhouse I plant in pots or ring culture pots (with no bottoms) as I do in the slab based one. So no difference there. However, I have a peach tree growing in the soil based greenhouse and I would not be able to do this in a slab based greenhouse, although you could have a vine planted outside the greenhouse and growing through a hole in the glass. The peach is in a greenhouse to prevent it getting peach leaf curl, protect the blossom which comes early in the year and to accelerate ripening of the fruit. If the greenhouse was bigger, I would plant an apricot and a nectarine in a greenhouse too and for the same reasons.
I have a lot fewer weeds in the slab based greenhouse, although they find their way into the cracks between the slabs. I have just cleaned the slab based greenhouse and only found one weed which was easily dealt with using a slab weeding tool.
Although this is a bit of a chore, I replace the topsoil and the woody chippings in the soil based greenhouse every year,especially if I have been growing tomatoes. It mitigates against disease and I find that it is well worth doing.
So I think it depends on what you want from your greenhouse.