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The debate between running 1pc and 2pc rotors has raged on for years. There are clearly benefits to each.
1pc: usually very cost effective pricing for replacements. Local auto store in many cases and easy to find a replacement if you need on in a pinch. Generally 4-6lbs heavier in mass but most of that is in the center and has marginal impact on performance given a full weight car. Most are non directional cast and flow less air at speed than 2pc. Can become heat saturated due to iron hat and center possibly transferring more heat to the bearings as well. Many can be easily turned if they have warpage/deposit issue as they are usually sold 'plain' - without holes or slots. Easy to install- remove from box and fit. Replacement costs; $80-125ea depending upon the car and supplier.
2pc: One time, more costly 'buy in' vs 1pc. Can be upwards of $8-900pr at times if you only purchase them without a "complete" kit in which case you'd spend perhaps $1700 for all of it- including the calipers, pads and hoses. Sold usually as a "complete kit" given the buyer the benefit (arguable) of obtaining the above parts also. Castings are always directional at this point. Improved air flow at speed. No gains for the dd at this level however. Generally 4-6lbs lighter (yet often larger if sold in complete kit) than 1pc making for more effective weight and lower inertia issues. Improved heat management for better cooling as well as the air flow, possibly lowering bearing temps too. Many shops refuse to cut or turn them due to the interrupted cut caused by gas slotting or holes. (can be done, just slow or by grinding). More complex to service often requiring hardware and assembly time. Replacement costs; $250-300ea range.
The most common question is "if the 2pc is better (arguably so) is it 3X as better given the cost? That's hard to quantify. Given the quality of the rotors from Wilwood (think Spec37 parts) and other top suppliers....I'd say the life cycle is easily equal too, most likely double but questionably 3X the life. That's so subjective based upon the pads being use, the track use, the street use, how one drives, even if the car has an automatic trans in it. Overall I think you'll find the benefits of the 2pc are there for the more hardcore users but at the "cost of doing business". The budget guys however can get by on the one piece quite well.
Seeing that it's harder to find an exact replacement 2pc of the same diameter that alone makes the comparing of them tough. A 1" larger disc will always operate more efficiently. Perhaps even a 1pc larger vs 2pc smaller. Mass is mass and using it well is the key.
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