PPW and I day, or similar at the other SAs, have different opportunities and challenges and are also done slightly differently at different SAs.
The good part of PPW is the parent is thinking I will have the good part of 3 days to spend with my child to hear about what happened and all the details. The reality is your child will be very happy to see you but will likely want to talk less and share fewer details than the parent would prefer. The child is also likely to sleep. Alot! In the midle of the morning. In the middle of the afternoon. After going back to the SA and sleeping overnight they will want to sleep some more the next day. It is likely that PPW will be the occasion where you witness your child sleep more than you hav ever witnessed them sleep before. But its not to be missed if at all possible.
On I day you will have far less time but it is also a far more stressful time for the child. On PPW the emotion is "I made it." On I-day the emotion is "what I have I gotten myself into." The support of a friend or parent is usually - but not always - welcomed. This too is not to be missed if at all possible.
If you can only attend one, for me it would depend on my child and what I thought would be best for that child. For either, if unable to attend, you can be connected with a local sponsor family who will make sure you child has a plac to go or somebody to talk to.