IMO you can't go wrong with either. However, that being said. Most or at least the majority will tell you to get the Griot's Garage 6" and IMO you can't go wrong. It is a more powerful machine and can have the same adapters and pads like the PC. Additionally, it has a lifetime warranty. Your correcting time will be faster (my assumption) than the PC7424XP.
People on this site rave about the GG 6".
Now a little devil's advocate...I HAVE THE PC7424XP!! Blasphemy!! It has been a wonderful DA for me and I still use it for sealant/waxing. I did upgrade to the Flex 3401 as I felt the PC was on the weaker side of correcting and polishing. It still has a purpose and will always be in my arsenal for detailing.
If I had to do it all over, I would lean to the GG6" as (from what I read and hear) it is the more powerful machine and you could probably correct faster. As for amps that would be a "no-worry" for me.
Whether you choose the PC or GG as a starter...are good choices. You will get many other opinions but do what you feel is the best!!
One thing is for sure...once you start using a DA (any brand) you will be telling yourself "WHY DIDN'T I DO THIS LONG AGO!!"