Hi.
The major taxi firms use a fixed rate from ARN to anywhere in the city, SEK 450-550 or so. I don't think there's a late night supplement, but I may be wrong.
The lanes closest to the curb at the taxi stand at ARN (assuming you're arriving at Terminal 5) are reserved for the majors, and I'd recommend using one of those to minimize the risk of being scammed.

The train is faster than a taxi, but at that time of night there won't be much traffic so a taxi should not take too much time either. If there's two of you, definately use a taxi for convience.

Enjoy Stockholm!