Of the two the Tamiya Fletcher would be my choice. If you are doing something from the South Pacific, then I would assume you are speaking about the period August 42 to February 43, the Guadalcanal Campaign.
The Taniya is a round bridge Fletcher, from which you can do all of the Fletchers in the Cactus Strike Force, with very little trouble. What I would fo first though is get a copy of Rick Davis' book "Round Bridge Fletchers". What you will discover is that of all those early Fletchers, there was no two that were exactly alike. Not great differences mind you, but enough that you can make every model just a bit different from its sister, should you choose to do more than one as I have. My collection includes Radford, Nicholas, O'Bannon, and DeHaven, and I have not finished. I plan to do a couple more this year.
The Tamiya offering is not without its faults, but, in my estimation, it is a far better starting place that the Trumpeter. The Trumpeter The Sullivans is either a mid war or late war (your option) Square Bridge, and I know of none of them that served in the South Pacific during the period I cited.
Most if not all of the Tamiya's failings can be corrected with aftermarket products by Black Cat, and Model Monkey. You also need to obtain some PE doors and railings. They are readily available in a number of Fletcher dedicated and generic destroyer sets.
Good luck with your project, and I hope you will post photos here of your progress.