Jim,
I know the Dynam is more than you need but if you look at the weight of the 2 , .88oz. for the dynam and .85oz. for the Mystery I would use the Dynam as the difference in weight is negligible and that controler is proven. Its about $1.00 more but at least you will know it will work the first time and Will is familiar with those so it will make it easier for him. Remember the biggest drawback if you order from dealexteme is it can take anywhere from 7-28 days to get to you. There are vendors on RCGroups.com that also sell these for a few dollars more with reasonable shipping charges if ou need one sooner.