If the price difference doesn't affect your length of stay or ability to afford your vacation, I would pick the Contemporary simply due to the monorail. That just makes everything so convenient. You'd have access to a character meal right at your hotel, nice pool and marina area (definitely do the Pirate and Pals cruise if that's your kind of thing). But seriously, getting on the monorail at midnight and being whisked right back to your hotel is awesome. If you have small kids, its a no-brainer. So much easier to take them back for naps and an afternoon swim when its scorching or escape the afternoon downpour that happens in Florida.
Both months are generally hot and crowded, except I suppose the end of August is better as people return to school. I'm an "on-property" girl though. Its Disney, so either way, it will be great.