difference is max flow volume of 1850 vs 3170gph, minimum sustained flow is 30% of those numbers. The 6101 replaced both of these models
quick list of controllable streams
6055, 6000, 6100, 6200, 6101, 6201, 6301, 6105, 6205, 6305
There are standard powerhead type controllable tunzes but I am not as familiar with the model numbers