I am not going to say that if I am a Martial tank I would rather have AR over Avoidance. But what if you are an expert light fighter. No magic to boost anything. Would you rather have the +1 to avoidance of the Masterful Defense Talent, or the measly +1 of a Leather Armor?

It's very dependent upon what you're fighting I've found. Some enemies are going to hit you regardless of your Avoidance and some enemies are going to hit so hard that a 1 AR isn't going to do much. It's more about what all can you 'stack' in either Avoidance or AR as to which is better for you.

From a purely mechanical standpoint, the answer is, "it depends."

We're dealing with a bell curve. Whichever stat you can push further up the curve (beyond the midpoint) is the better one to adjust. In most instances, shoring up a weakness doesn't benefit you as much as bolstering a strength. This isn't a hard-and-fast rule, but a general rule-of-thumb.

Scott

If you going to be a light fighter that is not martial, but neither any type of caster (i.e. expert), then you have very few choices to boost your AR. You will start out with a 1 or a 2 AR. You Then have to take the feats Medium and heavy Armor Proficiency to get good AR. However if one of the keys to your character is mobility then you end up taking bulk penlites.
I assume if you really want to cheese it out you could be a light fighter. Take Medium Armor proficiency (Kio Hauberk). Take Heirloom (Kio Hauberk), thus using two of your starting Talents, and then you have 3 AR with no bulk. First opportunity get an unassailable rune to bump it up to 4 AR with no bulk. That is the only non-magical non racial (natural armor) way to get any AR.

So maybe taking masterful defense and maxing out insight and prowess would be better?

Any other ideas how to get good AR without being a martial light fighter.

Why no bulk? Heirloom grants you exception armor but I'm pretty sure exceptional armor explicitly says "minimum 1" on the bulk reduction (but I don't have my book in front of me).

You are just making my problem worse. Do I settle for restricted movement for little benefit (1-3 AR) Or do I try to get my Avoidance up as high as I can?

If I cannot adequately get by with little bulk (no more than 2) I usually avoid armour in general and go for avoidance instead. Masterful Defence + Shield seems the best way to go for Experts IMHO.

Or you could go with a Martial build, and use Advanced Armour Training to ignore that one point of Bulk

Martial does not have the necessary skill set for the character concept. Now if that one forbidden talent was legal . .. but alas we are stuck were we are.