The E190s have LiveTV just like the A320s do so that part won't change. I'm not sure if the LiveTV service will work all the way to CUN because you might leave satellite coverage range, but I don't know.

As for pricing, buy if you're comfortable with the price. If the fare drops you can always have them re-fare the ticket and you'll get a jetBlue credit in the amount of the difference for future use.