Magician's performances are seriously lackluster. I wouldn't take it into DD. "Par" for a medium combatant seems to be around 9/10 BAB including stuff like performance, mutagen, and arcane pool enhancement, or "assumed" buffs like Divine Favor and while DD hits that ratio for its own levels you'll be behind if the bard part doesn't. The only real advantage to be seen is getting Enlarge Person on your spell list.
Magician seems to me to want to be mostly a caster with the extra spells known and dweomercraft bonus on caster level checks and concentration checks. Magician's abilities seem built for counterspelling, dispelling, and UMD and not very good to go into a melee PrC like Dragon Disciple.
If by "Straight bard might work too," you mean unarchetyped bard into DD then I'd do that. Inspire Courage is what makes a bard hit as much as the other 3/4 BAB classes. It also will probably help ray or touch attack casters more than Dweomercraft. Since touch attacks and rays can be selected for the weapon focus tree and most people allow weapon specialization in them they should also count as weapons for the damage bonus from Inspire Courage. There's no qualifier on the bonus to attack rolls which certainly will benefit them as well as non-ray ranged touch attacks.
My instinct would be to build as a bruiser with the 18+2 in strength, a 12 in con, 13 in int and dex, and 15 in charisma. Or even a 13 in charisma and 15 in dex for the AC. That's assuming, of course, that the DD is because the player wants to go into melee as the party's tank with that nice shiny d12 hit die. I wouldn't mess around with any other classes. White Grotto looks like it will offset the DD caster level losses, but not more than that.
If the magician part is more important than the DD part I'd go for a dispel monkey to leverage your bonus to caster level checks and your spellcraft bonus and use cheap wands as your primary contribution to combat to leverage your UMD bonus. Those seem to be the archetypes strengths and not building towards them would be a bit of a waste. This would involve building basically like a caster.
