it all depends upon what you want to do.
If you want beautiful white sandy beaches with turquoise water, good snorkeling and really good shopping then pick an Eastern Route.
If you want something a bit more active with swimming/snorkeling with the StingRays, climbing water falls, river tubing, zip line in the rainforest, awesome snorkeling, beaches, Mayan ruins then pick a Western Route.
FYI - august is the Peak of hurricane season, you need aware and be a bit flexible in your schedule - departures could be affected, ports could be changed. Make sure you take out trip insurance- trip cancellation/trip interuption, baggage and medical evacuation are the coverages you will need.