Welcome to FlyerTalk.

The Ilikai is at the west end of Waikiki and the Marriott is on the eastern side, more central to the activities within Waikiki. I don't find the area the Ilikai is in very interesting but it is convenient if, like you say, you plan to do a lot of sightseeing.

You might some useful information in the Marriott forum as well as www.tripadvisor.com.