I've always thought that three Leman Russes was a little lame, but it's undeniably extremely good. Each can cover a portion of the table, and when equipped with three heavy bolters and a heavy stubber, they can lay down more accurate anti-infantry fire than firing the battle cannon in some cases, particularly if the target has spread out. I'd bring the sponsons, because they're not expensive and they offer an option that tends to have its uses. Even if you never fire them in a given game, they're a good investment.
A squadron of Leman Russ (not a vehicle squadron by game rules, but three Leman Russes) can be a game winner just by virtue of their battle cannons. Given good scatters, there's no reason for an infantry advance to ever reach your lines in sufficient numbers to be a serious threat. Deployed in an echelon formation (either left or right, depending on which flank you put them on), you can cover the entire table with an armored strongpoint-- the lead vehicle covers a forward arc, the middle covers the flank and half of the front, and the rear vehicle covers your own lines, as well as still being able to cover a portion of the front. As a Space Marine player, one of the things I really hate seeing across the table is a squadron of Leman Russes deployed in an intelligent formation, because I know there's really nowhere I can hide from all three of them at once, and the Guardsmen aren't going to be coming to me.
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