[SixOfSpades]

Fighter/Mage
Advantages: Access to both the Fighter and Mage ToB High-Level Ability pools, more weapon proficiency points, more hitpoints, better Saving Throws, better THAC0, no long period of being handicapped waiting to regain your skills, can use any weapon or armor, potential of playing a Fighter/Illusionist.
Disadvantages: Lower number of high-level spellslots, takes longer to access those spell levels, cannot go higher than 2 stars in any weapon.

Fighter->Mage
Advantages: Can Tank during the early game and be a high-level Wizard during the late game, gains Mage levels rapidly, can achieve Grandmastery in any weapon, can wear armor.
Disadvantages: Virtually handicapped while waiting to regain Fighter half, no Fighter HLAs, rather poor THAC0 and Saving Throws.

Kensai->Mage
Advantages: Can Grandmaster in any melee weapon, okay THAC0, can be very powerful in late-game major battles, gains Mage levels rapidly.
Disadvantages: Virtually handicapped while waiting to regain Fighter half, no Fighter HLAs, rather poor Saving Throws, can never wear any Bracers, armor, helms, or shields, and can only use Throwing Daggers (cheesy) and Throwing Spears (most of which are incredicheesy) for permanent ranged weapons.

Mage->Fighter (Dualed at Level 2)
Advantages: Can use all Mage items (Amulet of Power, Staff of the Magi), can cast 2 or 3 Level 1 spells per day, can cast spells from scrolls (Stoneskin, PfMW) in full armor and mid-battle, can Grandmaster in any weapon, better THAC0, hitpoints & Saving Throws, and the game's AI treats you like a Mage, causing enemies to target you with spells like Breach and Ruby Ray when you're not even using any protections.
Disadvantages: Cannot choose a kit, no Mage HLAs, and some Usability errors will prevent you from using certain scrolls (some Stoneskin scrolls, for example).