Like everyone has said above it varies with respect to the particular combination. However, from a purely min/max perspective many classes are very frontloaded (you get a lot of advantages at 1st level). So if you multilclass you can gain a lot more feats and faster saving throw advancement by multiclassing. The place where multiclassing hurts is long term spellcasting progression. Stay inside Fighter/Rogue type classes for multiple levels and take just one level of a spell casting class to gain it's initial advantages.
With regard to a Fighter5/Cleric1 vs. Fighter 6. I think the F5/C1 is better, particularly if you choose a god with the right domains. Kord, a fighter god, can give strength and luck domains. Very nice to be able to reroll once per day or gain the 1 round boost to your strength which is based on character level not class level. You also can have command up your sleeve or augment healing or heal yourself if separated from your group. And, there are other great domain abilites as well, so don't forget to factor them in to the power gained by the cleric level.
A down side of this is that you lose your second attack as the cleric has a bab of +0 at level 1. Relative value depends on your immediate needs.
With regard to a Fighter5/Cleric1 vs. Fighter 6. I think the F5/C1 is better, particularly if you choose a god with the right domains. Kord, a fighter god, can give strength and luck domains. Very nice to be able to reroll once per day or gain the 1 round boost to your strength which is based on character level not class level. You also can have command up your sleeve or augment healing or heal yourself if separated from your group. And, there are other great domain abilites as well, so don't forget to factor them in to the power gained by the cleric level.
A down side of this is that you lose your second attack as the cleric has a bab of +0 at level 1. Relative value depends on your immediate needs.