Peter,
I was there for a couple hours last weekend. I was mainly interested in raptor activities but did not see any while I was there. However, I did see a fair number of egrets, coots, and other wading birds, including *one* roseade spoonbill and *one* white pelican.
In terms of the two lenses you mentioned, I think the 150-600mm would be more suitable. I am sure the 500mm would give you better IQ; however, several of the wading birds I saw were pretty much right next to the drive which may give you some issues with a fixed 500mm. And then some other birds were definitely far enough that the more reach would be better. I did not see any fast BIF actions that would give a fixed prime some advantages.
It was *very* cloudy when I was there so sun direction was not an issue (in fact, we were rained on, too). Consequently, I forgot to check if morning or afternoon would be a better time of the day to visit. Having said that, the drive was curvy and changed directions enough that morning/afternoon would give you "equal opportunity".
Good luck, and have fun.
Linh