Which would be a better location? The Fairmont is nicer and a splurge, worth it if it will make our 3 days in Prague more convenient.
They are less than ten minutes apart by walking, and appear to also be approximately equidistant from the main square.
But the two hotels exist in different universes - Hastal is a smaller, European style hotel [we had a wonderful stay there] and the Fairmont is - well - a Fairmont. So the choice is between your hotel experiences. Fairmont is closer to the river; Hastal doesn't have views.
The center of Prague is so compact that the Fairmont location won't be much more convenient, unless you intend to spend a lot of time on the castle side of the river.
We stayed at the Hastal Hotel and loved it. Easy walk to the main square.