Shoebuy has better discounts, but Zappos offers free overnight shipping. Since they both offer free returns, I'd order from Shoebuy first (asap) - you will get the tracking info sent to you - if it doesn't look like they will be there in time, you can always place an order at Zappos (if you order in the AM, you get them the next day!) and return the shoebuy (no cost for return shipping) when you get home from your trip.
Fwiw, I ordered a few pairs of shoes from shoebuy Thursday and they just shipped yesterdy - it is still not showing an expected delivery date on the UPS tracking website.
Another tip - if you need to order more than one pair (or are trying more than one size) place separate orders - not only will you get an additional 10% off subsequent orders (no code needed, automatically deducted), but each pair ships when it is ready, rather than the whole order "waiting" for the last pair. Also, ifyou need to return anything, you can send each pair back separately, rather than deciding on all the pairs in that order at once (since in the past, it has been only one return label per order). Hope this helps!