There are several primary differences between the ProPower 200 amp units, and the 250 amp Rhino series units. Both units are perfectly fine for daily use, with excellent output at idle. You can see full output curves here:
http://www.mechman.com/types.html Remember that the RPMS listed on the graphs is ALTERNATOR rpm, not ENGINE rpm. You need to divide the alternator RPM by your vehicle's pulley ratio to determine engine RPM; most vehicle have a 3:1 pulley ratio, so divide bt 3 as a rule of thumb.
The 200 amp is built with 50A diodes, aluminum rectifier plate, uses a conventional ouput stud, HD bearings, and HD regulator. It produces 125 amps by 800rpm engine speed, is made here in the US with 100% new parts, (not remanufactured) and includes a full birth sheet from it's dyno session. (a MechMan exclusive) 1 year warranty
The 250 amp Rhino series is built with 70A diodes, 3/16" Copper rectifier plate, Oversized positive and ground studs, adjustable voltage (on select GM models only) HD regulator, HD bearings and produces 150 amps by 800 rpm engine speed. Made in the US with 100% new parts, includes a full birth sheet and 1 year warranty.
Because both units are based on a custom machined LARGE case housing, overheating is not an issue, as it is with overwound, rebuilt stock rectifier small case units. (like you get for $240 on E-bay) Here is the difference between our units, and other high output companies:
http://www.mechman.com/size.html
Hope that helps, feel free to email me any questions, see my sig.