I like Holtby a whole lot more, for next year and the future (I don't imagine Andersen being the #1 starter a few years from now). Holtby is young and, like you said, on a rising team. I think ultimately it depends on how tight your cap-space is. If you can pull it off, Holtby is a good bet for the long-term. If you'd have to sacrifice too much, Andersen will be just fine for next year and perhaps the next but I'd recommend looking at trying to get something more long term.
In conclusion, I'd go Holtby if your current cap space allows for it and doesn't sacrifice too much up front. He's worth the extra $!