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3♥ wouldn't be forcing, 4♣ wouldn't be forcing and indeed, 5NT wouldn't be forcing. So the question would just be: Knowing that your partner has an 18-19 balanced hand with 5 spades, which contract would you pick? You are forced to punt.
The obvious answer is that 5♣, 6♣, 4♥ and 6♥ will all work some of the time and none all of the time. I am pretty sure that the OP is aware of this obvious answer. If his question would have been: "You are forced to punt, which contract would you pick?", he would likely have phrased it like that.
The way I read the OP is: We don't have any agreements about 1♠-1NT; 2NT auctions (not even which bids are forcing). What would you bid in that case?
Josh' answer is that he will punt, preferring the risk that he picks the wrong contract over the risk that partner will pass 3♥. Ken's answer is that he will bid 3♥, preferring the risk that partner will pass over the risk that he will pick the wrong contract.
My answer would be that in this case, I wouldn't evaluate what cards I am holding. I would evaluate what kind of partner I have. If I think partner is a newbie (or an 'expert'
) I will bid 6♣ and apologize if it turns out to be the wrong contract. Maybe afterwards, I will give him the tip that the best way to play after a 2NT rebid by opener is that everything is forcing to game, except for Pass. (I will do that when 6♣ was right and when ot was wrong.)If I expect that partner has a basic idea of priorities in bidding, I will bid 3♥, since I expect partner to bid on, even if he isn't sure whether my 3♥ was forcing. In this case, I will apologize, but there won't be a tip. When 3♥ worked, I figure that he knows what he is doing. If it doesn't (since he passed), his idea will be that I don't know how to evaluate a hand (making a NF bid on a 0616!), so he won't listen anyway.
Since we play 1NT as forcing, I am going to assume that partner is not a newbie and I'll bid 3♥ at the risk of finding out that partner is an 'expert'
.Rik