Technically a alloy head would be better for the reasons Muskrat has mentioned but in reality it doesnt matter that much unless you want to start exploring the outer ends of the bikes performance. The iron head will perform and last perfectly well so use it unless you want to spend the money repairing your alloy one.
As for the story that small fin heads run hot, a load of rubbish, old wives tale. Maybe if you put 10-1 pistons in or something like that but under normal use they are perfectly ok. Mine have been subjected to some very harsh conditions and I havent had any top end troubles yet. My plunger A10 (and the A7) regularly gets ridden in the summer on days up in the 40 plus degree C's, usually reasonable distances and always at 65mph or round abouts and it (they) havent given me any trouble (yet). Mind you the bottom end has just crapped itself on the A10 but thats another story.