They are both manufactured by the same company, Microchip, and these days, with C compilers (and others) the hardware can be reasonably transparent, so it is often not an issue like it may once have been.
You could use either family, both are capable. Free tools are available for both, and there is plenty of support. Most people argue for a family based on what they commonly use historically, often without looking at alternatives, so I won't fall into that trap, you need to make some of your own choices. Having said that, have you looked around for information on other robot projects, it may help you to get some perspective. There are probably some based around some of the Arduino family of boards for example, which off loads the need to do your own processor card unless you really want to.
It is also possible that something like the Microbit board may work for you, which uses neither of the processor families you mentioned, so it doesn't pay to get too hung up on the AVR or PIC issue.
You need to add up the number of pins for the functionality you need, for the sensors you plan to use, general purpose digital I/O, and the PWM capability of the processor for driving the motor. If your sensors need ADC's or DAC's, how many bits do you need, and what speed. Do your external devices work from 5V or 3.3V, or are they mixed. You may need to processor that will work on either voltage and need some level shifting either way. There are many questions like that that need to be answered, hope I haven't overwhelmed you, a bit more information about your sensors and your programming skills may help in answering further questions.
Cheers,
Richard
