Land is defenatly easier to balance... At least, you can "decide" on where ground attacks will be able to come from, while island maps usually are (in my oppinion) a bit more unbalanced... Zerg and Terran can just get easy transports quickly enough, but Protoss has to tech pretty high and spend a great account of minerals on shuttles, then load them. Arbiters are even harder to get.
My 2 cents. I made 1 island map... Took me the longest time, lotsa lil details, but it suked. Cuz its island
. Just kidding...
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The air balancing as far as race goes, i thnk they should all be about the same. The protoss MAY have a slightly weaker transportation system but their air strike force would compensate for that.
Yes you can decide on where ground attacks can come from, but it's much much harder to balance that out; each pathway of destruction must be evenly spaced out and spread among the map as well as the decoration and realistic qualities they possess. Not even blizzard's maps like LT have it perfectly balanced.