There was a long discussion about this here:
http://forums.mikeholt.com/showthread.php?t=141592
I disliked the tables in annex C ever since they introduced them (1996 if I remember correctly - maybe 1999). They added an additional book to an existing book with all of those. I find it just as easy or even faster to just use tables 4 and 5 than to find the right table in annex C. If you don't have all same size conductors you need to use tables 4 and 5 anyway if you want to be accurate.
I assumed the XHHW-2 diameter and area sq/in were actually smaller than THHN. I could be wrong.
However, I think XHHW-2 is more common in compact Alum conductors and is more available in stock.
I looked at Southwire's site and it looks like they only have THHN with their "Sim-Pull" insulation. Have you priced that vs. XHHW-2? I think you pay a premium for that insulation.