The map generation for pre-mistlands and after mistlands is a LOT different. Mistlands in the beta generates easily triple the amount of mistlands on the map, and probably more, and reduces the amount of swamp and attendant crypts on the map (from a max of 400 crypts down to 175). The biomes are much more a collection of concentric rings with mistlands. (Not entirely of course, but there's a strong tendency towards rings) Only you can decide which version of world generation suits you more. For anyone in doubt I suggest you generate several worlds now to save for later so you have old-style worlds available should you ever want them.