I would definitely go with the growth formula for the simple reason is PROTEIN! The other is higher is fat which will help him with weight, but protein will help him to grow to be as big as can. It may require you to feed a little bit more of this feed than the other to get the weight up but you can also go out and buy a weight builder top dress to add to his feed. You shouid not have to worry about him getting "hot" off this feed either because it is a low starch and sugar feed. High protein feeds are good for muscle developement and bone growth, especially if they are fortified with vitamins and minerals.
In the cadence feed, it would be good for a mature horse that is a harder horse to keep due to the high fat content. This fat content comes from the ultimate finish that is in the mix. This feed is actually designed for mature horses that do performance work.
As far as switching grain on horses to be bad, this is not true, if done correctly. Feed changes should be made gradually over at least 7 days, I do 2 weeks when I change. As long as you are not changing feeds everytime you turn around, then it is not bad to change a feed. Newer and better feeds come out all the time, and maybe the feed you are using at some point and time will just not suit the purpose anymore and you have to find something better for the horse themself. You just need to find a feed that works for the horse and within your budget and you will both be happy. I would allow him access to all the grass he can eat still and if he will eat it, hay also. I know in a previous thread you said he would not eat much hay if he was outside.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"