Both sets of dates are well into the summer season, so in general, crowds will be high, but the parks will be operating on long hours with full entertainment. Regardless of which dates you select, I would go expecting long lines and high crowds. The new parade, fireworks, and WoC should already be running nightly.
July 4th is traditionally one of the busiest days in DLR, but there is a special fourth of July fireworks show! I've always thought that seeing the special 4th of July fireworks would be fun. Also, 4th of July is going to be on a Saturday this year, so crowds will be high from fairly early in the morning. DLR is having a major anniversary (the 60th) on July 17th (Friday), but I don't really know if it's going to have a huge spill-over effect on the days leading up to the 17th. I'd be inclined to say that it might be slightly better to go during the weekdays (Tue to Thu) ahead of the anniversary rather than over the holiday weekend.