if you look at the block diagram of what's inside a switching power supply controller, it's generally reasonably simple, such as using a triangle wave and comparator to generate the PWM signal, and then some extra stuff for current limiting and whatnot... I don't think a PIC is really suited to that - using a clocked digital system which can only respond at a rate on the order of its instruction clock speed seems to be a big limitation compared to an analog system which has no such restriction... the slower reaction time of the PIC would first of all limit the PWM frequency, and also increase ripple, since it wouldn't be able to instantly respond to a changing voltage...
*edit* I've now read your project log a bit, and seen some of your reasoning...
Well, it seems to me it would be better to use a combination of analog and digital... the analog system seems like it would respond better to ripple, but you could have it heavily integrated with the PIC for things like determining the PWM frequency. sort of like giving a microcontroller access to the inner workings of a regular switching converter controller IC, you'd get more flexibility; only you could build your own.