I think I'd rather be cool than have it quiet. 13.5k BTU's is still 13.5K BTU's, no matter how efficient they are.
If you have the option, go with the larger unit. You'll never regret getting MORE than you need, but you'll always regret getting less than you need and having to upgrade later.
Also, you may as well go ahead and add a second unit---same argument as above. Plus, you never know where you'll be taking your vacation one day!