Courtesy of SPIN farmer Mike B, Iowa:
I don't think you can go wrong with a good heat mat. You are paying for the ability to be able to control the temp. to within a few degrees, as well as for the amount of space you want to heat for seedlings. I'm growing on about 1.5 acres and have a heat mat that
is 24"x 48", and it was a great investment. I went for the slightly larger size ($150.00) knowing that I could always grow into it.