Sliver overlord is 7/7 compared to Karona's 3/3 (6/6 if you call Avatars as your creature type) for one less coloress mana. You do need 3 other slivers to make it break even in terms of the buff, but it has the upside of NOT GIVING THE OPPONENT YOUR COMMANDER, on their turns. Even then I tend to prefer the Sliver Hivelord anyways. An indestructible commander along with all your other creatures being indestructible can be quite obnoxious.