I generally only buy growlers of stuff that isn't sold in bottles. Unless the growler price is excellent and I can't find a bottle of it. For example, Founders Sweet Repute sold for about $19 for a 26 oz bottle, I got it on draft at $14 for 32 oz.
And I generally limit myself to 32 oz growler fills since the product starts to degrade the minute you open it. Even 32 ozs is a stretch on the high abv beers.
Bottom line, if I can get it in a bottle, that's what I'm choosing. If I can't, the price has to be pretty sweet. What is pretty sweet? I know it when I see it. $16 for a Three Stars beer isn't sweet (in part because anything after 32 oz, the value declines for me).