Opener rebid priority
With SAYC a balanced hand would normally be bid as follows:


13-14 points - open 1 of a suit then rebid 1NT
15-17 points - open 1NT
18-19 points - open 1 of a suit then rebid 2NT
20-21 points - open 2NT


Your rebid should be 2NT. If you bid this, your partner has a very good idea of what you've got and can then decide how best to proceed.

It's also worth noting that when your partner responded he should have been "bidding up the line". So if he had the same number of hearts and spades, he would bid hearts rather than spades. A heart fit is therefore unlikely.