Jess: 21F is cold!--though it's often colder here... I would shoot for 10mm poly, your pocketbook will thank you for the extra insulation that it provides when it is super cold out there. I always think that a bigger heater is a better heater, you would rather have it warm right now, rather than struggling to overcome the cool, but given the size of your greenhouse, either of these should suffice, unless you are hoping on running a hot house (85day/70night), then I'd go with the bigger of the two...
Given the choice of the two, I would go for higher sidewalls (you will be able to make better use of bench space, as spikes on the rear of the bench will not have as much of a chance of hitting the slope of the roof, you will have more flexability with bench height and you will not have to stoop should you decide to run an aisle on either side of the benches in the house). You never mentioned, what is the door height for the two?
Adam
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