I really have no clear answer, but there is lots to consider here.
Do you have a rule that limits rim width to tire recommended size? If so, 15.5's go on a 14" rim. If you wanted to use 14.9's you'd have to narrow your rims.
If you can use any rim width you like, my personal preference is 14" or 15" for 14.9 tires, and 15" or 16" for 15.5 tires. So if you didn't want to widen your rims, go with 14.9's.
I also like the tallest tire you can run, so another vote for the 14.9's. I think that these would work well to get as close as possible to your 3 mph limit if your rpm's are limited. If you can run 20% or more rpm's, the shorter 15.5's might be the better choice to keep your revs up.
Lastly, both are a great tread for pulling, but i think the super all traction's have the edge. If there was such a thing as 14.9 super all traction, that would be the pick for sure.