I think the #1 consideration is location, and I find it a little surprising there's no mention of that in your post. You might want to do some more research on Cartagena and where these hotels are located, particularly the Conrad. And read the Hilton thread, which has quite a bit of information on the property, location, and beach.

btw, if your goal is a Caribbean beach vacation, with white sand and blue water, Cartagena is a very poor choice.