Thank you for sharing the details. A few comments:
interpals has much much more active members.
Interpals lies about how many visitors they get, they have several times less than what they advertise (albeit still definitely more than PPG thanks to their #1 spot on Google). Well to be precise, I don't know for sure whether they lie or they have a very serious bot problem. Bottom line is their numbers are greatly inflated.
Interpals has an more accurate way of showing language levels. Interpals has much much more languages.
I'm not sure I understand this, presenting 4 levels instead of 3 doesn't add real value. Also, having more languages in a list (which PPG references 184 languages as of yesterday) doesn't mean you will have more speakers of said languages
On interpals at the side everyones profile there is a map showing in what city they live
On PPG you can list on your profile what cities or countries you've visited to show on a map, too (not just your current city)
Interpals gives the option to delete conversations.
You can remove conversations on PPG ("Archive conversation"). The reason why it isn't called "delete" is because if you receive a new message from that person, you still have access to the previous messages, which can be quite useful