PC:
--Pros--
- Faster
- Can do leagues more, in terms of performance
- Loads cheaper
--Cons--
- Not good for people who've never used any kind of computer before
- More potential to become infected with viruses (nevertheless for a very substantial reason)
- More hardware (this can be good in some cases, though)
Mac:
--Pros--
- User-friendly
- Good for graphic designers and audio/video editors
- Not nearly as much hardware (can be a downside in some situations, though)
--Cons--
- Unbelievably expensive
- Lacking in some vital performance (although as others have said, I highly doubt your grandmother is going to be playing/using graphic-and-processor-intensive games/programs)