I've shot them both ways! And really...it might just all depend on what time of the day you are looking to have the session.
My first couple did their TTD session EXACTLY the way you are talking about. We got the resort in the evening, and early the next morning (around 6am...if I recall) Becky and John met me on the beach for about a 1 and 1/2 portrait session. It worked perfectly, since by the time the family and friends got there...they were ready to be tanning poolside and NOT dragging lace through the Carribean. lol.
That being said...they ALSO shot with me during the morning time with the sunrise while their guests would have been asleep anyways! So that might be an option. Ask your photographer if he/she can get up super early to get the sun AND get the shots before your guests wake up. Then you can do the session on whatever day is easier. Of course, getting up at 6AM on your post wedding morning might be kinda difficult. lol.
My second couple did their session in evening during the sun set on the last day they were there. Most of the guests had left earlier in the day to catch flights out, so it was just their parents. The session lasted, again, about an hour and a half...the beach was definitely more populated...but we got some great shots!
All in all, I would find out from your photographer how easy it would be to fit in a super early morning session since that will probably fit your schedule easier...AND you get some great lighting.
Good luck and have fun!!