I figured this before reading your arguments:

If you have two aces, the probability that the one you pick is the ace of spades is 0.5. If you have only one ace, the probability that it's the ace of spades is also 0.5 (and if so the probability that you pick it is 1).

The conditional probabilities are 50/50 in both cases. Therefore the evidence has no influence on the prior probability. The probability of two aces remains 1/5.

As for the arguments:

I think argument 1 is oversimplifying. We'd have the same information as in scenario 1 if we'd just ask "is one of them the ace of spades?", without asking them to pick one. But asking them to pick one introduces a more complex conditional probability of an ace of spades, meaning the posterior probability can't be the same.

I'm not sure I understand argument 2. I can interpret it to be the same reasoning I employed above (in which case it is of course a brilliant piece of pure genius :P), or I can interpret it as the nonsensical thought that evidence you can imagine should have the same weight as evidence you actually find.