Personally, taking ToB into consideration... I would prefer dualling from a Fighter to a Cleric.
You can dual at level 13 and still gain
all your Clerical spells. You can have
Grandmastery in a weapon.
You start the game with 6*, you will gain another 2* before you dual.
[I]I recommend dualling at level 12, there is no real benefit waiting till 13, the only difference is 1 Hp and 1 THACO.

Up to you ultimately though.[/b]
A Fighter_Cleric with Grandmastery in Warhammers( Crom Faeyr) is a sight to behold. Slap a Regeneration spell, maybe add a Blade Barrier, and if things get hairy, retreat for a moment to cast a Heal spell.
Not to mention your Turn Undead which will cause the undead to... well, stay dead.
If you do not have ToB, then I recommend dualling at level 9 instead.