Cooler and humid or warmer and dry??

Sixty degrees should be good for running no matter what the humidity. What is the forcast high for the day? If you are planning on a 3-4 hour marathon it shouldn't be much more than 75 when you finish.

That sounds like running in paradise to me.

I try to do my long runs (8-9 miles) in the afternoon to avoid the morning humidity, but in the early mornings here its been in the low 80's with 90% humidity.

I just checked the hourly forecast. It's a 7 AM start, and the humidity is supposed to remain in the 90's through late morning. Temps are actually forecasted to remain in the low 60's with cloudy skies through when I hope to finish (around 10 AM).

I'm hoping that the lower temps and cloudy conditions will help to offset the higher than ideal humidity!

I hope you have a great race.

Alas, humidity does matter, especially later in the race when core temp rises and your body is trying to stay cool.  IMO the real danger is that the difficulty of cooling doesn't really strike until abou the same time that other issues begin to come to the fore (ie., around mile 18-22).

You know from your training that humidity affects your pace.   You would be wise to respect the lessons learned from training.  I know you were hoping to go out at around 6:50 pace.  You might ned to adjust that based you your experience training in the humidity.  Of course it is disappointing, but it happens.  You can't control the weather, but you can control how wisely you respond to it.