I forget Regret's and the Arby's main infantry composition, as I usually play UNSC, but I believe it is actually Regret who has the elite leaders, then Arby has a horde of grunts (besides the suicide grunts).

So the elite and brute leaders are essentially the same in terms of unit stats. However, the eliteLeader has these additional stats:

0.5
1
1

The last two being meaningless in the shipping build of the game I believe. The factor, I believe, is 'in addition', ie a 50% better at dodging rams.

Their true differences are in their 'tactics' definitions.

elites: http://pastebin.com/MvQW9m9U
brute leader: http://pastebin.com/Db2svsfd

The elite's PlasmaRifle vs bruteLeader's Brutegun shows that the brute is the more deadly squad leader (in accuracy and damage stats). Their grenade stats appear the same. You can analyze their InGarrison stats for any differences yourself.