As the title says. This is assuming you are not using a two handed weapon and are using a 16 dex at level 1 PC and a rapier or a 16 strength 1d8 one handed weapon as the other. Both fighters are using shields as well.
For sword and board, the basic tradeoff is pretty consistent across levels: The Dex fighter sacrifices 1 point of AC and grappling ability. In exchange, you get superior initiative, better Dex saves (much more important than Str saves), superior ranged attacks (longbow beats javelin in both damage and distance), and access to the universe of Dex skills.
You can also look at the offensive options for the two--the greatsword Str fighter and the dual-shortsword Dex fighter. At level 6, assuming an AC 14 foe:
Greatsword fighter: AC 18. Two attacks for average 10.833 damage each. DPR 21.667.
Dual shortsword fighter: AC 17. Three attacks for average 6.725 damage each. DPR 20.175.
So, basically the same tradeoff as above. The greatsword fighter has a
slight edge on damage at this level, but not an enormous one.
Starting at level 11, however, the greatsword fighter surges into a solid lead (DPR 32.5 versus 26.9, a 21% edge) and stays there, due to the fact that Extra Attack never grants extra off-hand attacks. Magic weapons can help the dual wielder a little, but not enough to close the gap. To add insult to injury, Superior Critical further favors the greatsword, since a larger fraction of your damage is coming from the dice.
So, to sum up:
- I think Dex-based rapier-and-board edges out Str-based longsword-and-board, but not overwhelmingly so.
- Two-weapon fighting is better than great weapon in the early levels, on par in the mid-levels, falls far behind at high levels. However, feats and other martial archetypes might change this.