We are from Southern California and we LOVE La Serena (well, at least the weather, but the town isn't half bad). Coastal desert just like SoCal - but MINUS the people!!!!
We also love Zapallar and Papudo (more like Northern California).
Not sure what you are into, but Las Tacas is a community out in La Serena - swanky (Chilean swanky I should say). Not sure if you are seeking creature comforts or a more authentic experience. I was born and raised in Colorado though, and I must say La Serena does not remind me of the Colorado front range. I also think it might be a little hot for the perameters you set out. I think you would be happier somewhere in the South of Chile like Pucon or anywhere in the Lake District.