As per the 5 units of 2 comment, that's unbuffed. In very few situations are you going to not buff your Gluttons and that's the reason you take them in a unit of 12. From a numbers standpoint, Bloodgullet is the superior Mawtribe to choose, well Bloodgullet offers many benefits. You get 6'' pile in instead of 3'' if you're near your general, which there's no real reason you shouldn't be. This means that unit of 12 can get a lot more models in during the pile in unless scenery prevents it, in which case that would be poor playing in more situations than not. It also means you get a 66% chance of landing +1 to hit from the Slaughtermaster, which will greatly benefit 12 models of Gluttons than 2 Leadbelchers. It also means you get 2 spells, one for the +1 attacks and 1 can give the enemy -1 save. Only a 50% chance for each one to go off. In addition to that, for a command point Gluttons and ONLY gluttons can reroll wound rolls, which skyrockets their damage beyond that of even Ironguts.

Considering those factors, leadbelchers can't even begin to compete, even if you take 2 extra bodies to compare 12 versus 12, in 6 units of 2.

Then you have to factor in the charges. If you're more than 3 inches away and you have 5 units of 2 leadbelchers, you have to make 5 charges. I'd say the average range you usually end up charging on a Alpha strike or Round 2 charge would be 6'', it's pretty rare to consistently get within 3 ''. The Gluttons only have to roll it once, which is a 50% chance and then they can reroll it, also gaining mortal wounds on a 4+ based on the value. The Leadbelchers in 5 units of 2 would have to roll 5 separate charges. If it's a 50% chance to make a 6'' charge then only 3 of them are going to get in.