What you are missing is that, if you were to donate in BTC, you would have to convert the PTS to BTC anyways.
Since you can convert between PTS and BTC back and forth. Your question is no different than asking if I have 1 BTC and want to donate, should I donate 1 BTC or convert to PTS and then donate.
The fact that you have some proportion of PTS and BTC makes no difference at all.
Essentially you are asking 2 questions in one:
1) When I donate, should I DONATE in BTC or PTS?
2) With my balance left, should I KEEP it in BTC or PTS?
The stuff you donate is gone regardless of whether it was BTC or PTS.