Both are good for metal.
Mahogany is going to color the sound and you will get enhanced bass and mids, more tone and more sustian, it is more forgiving of cheap pickups. however I like seymour duncans or bill lawrence "wilde pickups" xl500's Its hard to go wrong with mahogany,
Basswood is bright, and doesnt color the sound much at all. I has good articulation and yeilds a good crunch. However I feel like you get more of a progressive shred sound with basswood. Also good pickups are must for basswood. i like emg's or dimarzios. I would go play some basswood guitars to see if they fit your style since i don't think they are as versitile as mahogany.
your bridge design and material, nut design and material, and neck material will also have a fairly significant affect on the sound of the guitar. As well as the make and model pickup you select.