Mahogany is a warm sounding wood to make a guitar out of. Yes ash, either northern hard ash or the lighter weight swamp ash will both be brighter. Swamp ash is said to have more of a "Pop" as well, which is why it is used for Strats with single coils.
Warmoth added a tone gauge on their descriptions of tone woods which is nice to visually see how each wood compares:
http://www.warmoth.com/guitar/options/options_bodywoods.cfm
Also, what type of neck/fretboard are you using, which will make a ton of difference too.