What should I use: Hall effect sensor or current transformer?
Generally speaking, a standard current transformer would be easier to engineer for high accuracy. Hall effect sensors have to be specially compensated to achieve very high accuracy, but they can be used to detect AC and DC component current. If you don't particularly care about DC current flows - and I'm betting you dont - then surely you can find an off the shelf clamp on CT that will suit the entire range. I don't know what voltage you're operating at, but just make sure that the CTs rating prevents it from being saturated in the highest expected input current. Also, you could probably find a fairly standard 4 to 20 mA interface for the CT that could significantly reduce your overall development time, depending on your controller platform.