Ah, the grammatical shortcomings of the English language!
both of your variations are correct. Since English lacks ways to truly differentiate present and future tenses in indirect speech as well as adequate differentiation in the sequence of tenses, both are correct.
Direct speech: "Washington is the capital of the U.S."
Indirect speech (note tense of main verb): "He told (past tense) me that Washington was (same time as main verb) the capital of the U.S." This does not mean that Washington ceases to be the capital of the U.S. now, but it relates what was said at that moment, hence the continued use of past tense, but it does not negate future implications.
This is one of the main reasons Latin is such a great language. In Latine (Augustine, Dantxny, hoc vobis est):
Direct: Washington caput Civitatum Americae Foederatarum est.
Indirect: Dixit mihi Washington Civitatum Americae Foederatarum esse.
In this case you can't translate esse as anything else besides "was".