It depends.
How long is his torso? (sit him against a wall, mark where the top of his shoulders hit and measure from the floor to the mark)
How long do you want to keep him harnessed?
I will say that the Regent will harness longer, because it has 20 inch top slots and the Frontier has about 18.25 inch top slots (there is some debate about the exact measurements, but they are around those amounts).
BUT, the Frontier converts to a really good, really tall booster afterwards.
So they both have pros and cons. Unless his torso is already close to 18 inches, I'd probably say Frontier for longevities sake so he can use it as a booster afterwards. Although, I see that you have plenty of kids to pass this seat down too so... you could concievably use a Regent for him until he outgrows it, then buy him a dedicated hbb and pass the Regent down along the line... Sorry, I'm no help! I guess a lot would depend on his torso length.