Okay, so I'm in the process of planning out a Leman Russ-heavy army list. I want to shake up the different variants I use, too, so I've been trying to figure out which ones are best for what. Here are my general findings and I'd be curious to hear what you guys have to say.
1) Executioner: Because tons of plasma death is still pretty handy, even if you do lose a couple hull points a game.
2) Exterminator: High rate of medium-high strength twin-linked firepower is pretty clutch. Very reliable tank and cheap, too.
3) Standard Russ: Nice main gun for certain metas, doesn't need upgrades or sponsons to do its job. Middle of the road, really.
4) Demolisher: Great but for the range, which sucks. Might be handy as a Tank Commander (to zoom into position first turn and then unload second turn), but the medium range of it's main gun really hurts it, in my opinion.
5) Vanquisher: While the main gun looks okay in paper, the changes to vehicle damage tables in 7th Edition make it a pretty indifferent tank-killer. Give it a Tank Commander and Lascannon/Multimeltas, I can see its worth, but other wise it's pretty bad at it's main job.
6) Eradicator: Very useful for a very specific metagame. If you face a ton of hordes (orks, nids, enemy AM) or that one guy who footslogs a bunch of Eldar or Necron Warriors, this thing is gold. Otherwise, it doesn't have much use.
7) Punisher: I wish this tank were good, I really do. The short range, the lack of mobility, the terrible AP, and the mediocre BS make it virtually unusable for the cost, though. Even if you weigh it down with triple heavy bolters (the only way to fly, in my opinion), you're still only knocking down 3 marines per turn. That said, if you *are* in that metagame with the ton of light infantry, it might work out for you. I'd say the Eradicator would be better by half, though.