With the same gear and the same cadence the larger diameter wheel will make the trike go a longer way because of the diameter. It will be faster. However, being in a different gear or different cadence when riding together won't matter much. The only thing you will lose is high top speed from the larger wheel, and a lower first gear for hills with the smaller wheel. That probably doesn't matter either. The 20" wheels will accelerate a little better if everything else were the same. Also a very small thing. Smaller diameter wheels ride a little choppy as they drop in and out of low and high spots. I can feel it, some people can't or don't care. It's probably more important that the small wheel makes the whole trike shorter for transporting in a vehicle.
Decide by how they ride.

edit - I would pick the bigger wheel for a little smoother ride and better different tire selections available.