I'd go with the 5' x 6'. Here's why. Bulbs with reflectors put out light patterns that are pretty close to square. If you put the tables long ways in a 3' x 10' area, you've got two 3' x 5' areas...but the light that hits spots out of the area will mainly be reflected. That means loss of some lumens and efficiency. If you have a 5' x 6' area, you've still got two 3' x 5' areas...but the extra light will overlap more instead of either hitting a reflective wall or just going out into nowhere. I think you'd get better efficiency with the 5' x 6'. Just my .02.