I agree with the above, photoshop has a bigger audience, its easier to just paint, than to use vectors for many and when it comes to fantasy maps, photoshop makes it easy to make parchment, aging, scratching and all over doing prettifying stuff to the map ... I tend to use photoshop myself as it was the program I learned first and is most comfortable using - however my ongoing city project "Breakwater" (see signature) I am making in Illustrator and then I'll finish it off in photoshop later on.
But use what makes you comfortable at first - then you can always expand with more programs later on