What is this project? Are you making a signal generator? The PIC, if it is running with a crystal, will be rock solid. The 555 and TL494 will have temperature stability problems. Early on you said having to jump form 41800Hz to 41750Hz is not good. You want 41800 and the next step is 41801hz. If you build a 555 or 494 and want to control it to within 1 part in 40000 then you will need about 16 bit control. I do not think you can hold this type of oscillator to 1:40000 over hours or minutes.