Whats better for outdoor conversation?
 

2 lav mics, one on each character, or a boom? I would assume the lavs but I am not well versed in audio. With the lav would you take room tone later with a boom and mix them in post?

Better is both. As a general rule, the $1,500 shotgun mic is going to sound better (more natural) than the $300 lav as long as you can get it at the right distance.

Mix the lavs to one track and the boom to the other and try to get that boom near the sweet spot of about 16".

Obviously, there are circumstances where to have to use lavs...but, when you can...use the better mics.