There is no right and wrong in this situation. I personally put the date for the wedding and the AHR on the STD and the invites. Though I would love to have as many people as possible at the destination wedding, I don't want to mislead anyone into spending money they may not really have.
My fear of holding out on the AHR info would be: a guest is on the fence as to whether they can afford the DW and decide to come because that would be the only way to support my fiance & I and then be upset (and financially strapped) when they found out later there was an AHR.
Also - I have to say that having a smaller group attending our wedding (it looks like 24 or 25 adults + 3 kids + B&G) is keeping the stress way down financially and logistically. I originally thought I would want 40-50 ppl to make it more of a party, but having the smaller group means I can do more for each guest (like take everyone on a nice excursion).