Benefit of seperates as opposed to recievers is simple really.
A reciever has to box a multi channel amp, share power supplies, provide all decoding, and do it in limited space. ( theres more, but this is the crash course)
Seperates, Pre-Pros, are designed to limit internal noise, provide all the decoding etc, but with much better technology, and performance. (once again the short version)
The biggest difference between the two will be how you power your speakers. The pre-pro offers no power, and will utilize seperate amplification, which lets you decide how much power you need for what you are trying to accomplish, plus, since different amplifiers will sound different, you are able to choose from many different amplifier models to suit your specific needs.
Recievers (all in one) can have up to ( I believe there are 9 or 10 channel ones available) and are pretty user friendly, easy to hook up, and dont take up as much space by not having to have seperate amps etc. BUT, if a reciever says 100 wpc, it will not be 100 wpc. More likely it will only put out about 50-65 wpc when all channels are driven. (this can be a bit confusing, so I'll keep it at that)
Your "best" option, if that is what you are really looking to do, is to head out to your local dealer, and let them show you the difference in seperates, as opposed to all in one recievers. You will find that although some receivers have lots of options built in, they cannot compete with seperates, as far as complete performance is concerned, although some perform quite well for what they are.
Ive only given a small portion of what all the differences are, and there are numerous differences, but if your looking to build a quality system, it starts with seperates.