Well, there's a good argument for both sides.
To start off, cost. The IB is only 12 points cheaper than 4 Leadies with a musician, so it's not too hard to balance them off just the cost (as opposed to, say, a unit of Yhetees versus a single Sabretusk, etc. etc.).
Pros for double IB:1. Better odds of killing monsters/heavy cavalry. Even if one blows up, you'd still have a second.
2. Solid Chariot for late-game ramming power. They're good in a flank even if they can't fire.
3. Longer range; this can be important against some matchups, like other Ogres or Brets, who want you to get in close for them to countercharge.
4. Sturdier in combat; T5(6?idr) with a 4+ and 4(5?idr) wounds is hard for most things to chew through. And the few things that are designed to go after war machines will have a hard time facing off against these.
Cons for double IB:1. More odds for misfires, and nothing's more frustrating than having both cannons crack or explode.
2. Less CC attacks. This is assumed that it's either after the charge, or if the IBs are charged themselves.
3. People call cheese more easily
Pros for Leadbelchers:
1. Excellent chaff killers. This is especially important if you're running a Gutstar-styled list; it allows your big units to maneuver the field more easily mid-game for the important charges!
2. More shots/hits/CC-attacks. Overall, they'll get more shots, and whereas they'll do much less against monsters/1+cav, they're great for thinning out enemy hordes/character bunkers prior to the final slam. This has its own importances, too... Although, mind you, my 4-packs of Leadies do just fine against Silverhelms

3. No misfire chances

(This is a big deal for me, who had to suffer through the old book's rules, where I'd roll multiple misfires, get 2 shots, and have the unit wiped from the table lol).
Cons for Leadbelchers:
1. Mediocre Range. Depending on the game, you may only get one or two shots off with these guys before everything gets muddled. Then, after that, they lose a lot of their usefulness.
2. Not-so-sturdy. When they're in combat, they end up just being expensive Ogres. The key to them is to not let them get into combat until there's no other option

3. Blah BS. They only have BS3, but with the benefits of not having to get penalized for moving or multi-shot, they still end up only hitting on 5's or 6's in most situations, which makes them not really do as much in certain situations.
...Overall, I would probably cast my vote in with the Leadbelcher choice, but that's because I like their ability to clean off chaff, which is something that I think Ogres have a trouble dealing with otherwise. Sabretusks are okay for counterchaffing, but they're better served character/warmachine hunting.
Ultimately, though, the choice is up to you. Do you think, that in your meta, having two Ironblasters to deal with monsters or heavy cavalry will be good? Or, do you think, that having Leadbelcher cleaning up wolves and eagles and other chaff will be better for allowing your armies to advance? Maybe even try a test game playing with one or the other options!

33 Gnobs! Over 3 Packs of Trappers! Woot!
Gibblet, Nibblet, Crunchybit (from Slaughtermaster achievement)
New book WLD: 20/10/2 - Yay tourney wins and random game win!