I'd say "Mobitel"(012)has the best coverage in both urban and rural areas; it also has the highest number of subscribers.

I had started with the "Hello" provider (016), however, switched to Mobitel after a few years as friends and colleagues were unable to connect to my (Hello) network, particularly, after working hours.

Regarding costs, I would say, they are more or less similar; calls within and from Cambodia are definitely not as cheap as in/from VN. I once saw a comparison article on the very useful (Aussie-managed) website: http://www.bongthom.com.

Cheers.