There are no real drawbacks to either one. If you dual-class intelligently (not Dualing a Transmuter to a Fighter at Level 9), your character will always be more powerful than a pure-class character. To take your advantage of the Multiclassed Fighter/Mage: Sure, they won't be able to cast quite as many extreme-high-end spells per day (So? All you need to do is Rest more often), but they make
THE best Tanks: Stoneskin, Mirror Image, Spell Protections, etc (if you really want to go nuts, Simulacrum and Tenser's), then put their Full Plate back on and watch 'em go, and you can really annoy your enemies by having them cast spells from scrolls while they Tank.
Who cares if they can't cast 6 Time Stops per day when they're invincible on the battlefield?
The only Multiclasses I don't like are the F/M/T and the F/C/M, because they will never be able to cast
any Level 9/10 Spells.
The only drawback to Dual-classing is that you won't be able to get any of your first class's ToB HLAs. (Unless you Dual too late, of course.)