Don't underestimate the effort of designing a correct simulation of PIC hardware features for all processors. If you look at the various bugs produced by Proteus PIC simulation, you get an idea about it. Cooperation of MPLAB SIM with CCS is rather smoothly. If you prefer to use the CCS IDE, you need to import the *.cof file once in MPLAB, then it's automatically reloaded after compilation.
I also prefer MPLAB for in-system debugging of CCS generated code with PIC18 and PIC24.