Depends how many you have and how many you want for other things. For eg, PC is no longer an Amex partner so harder to top up via that route, whilst Amex to SPG still exists.

There is no straight comparison. Amex used to do 1 to 2 into PC and 2 to 1 into SPG, which implies that PC is the better deal. 16k SPG is 16k airline miles, 40kPC is 8k airline miles, so PC better to use for hotels on that comparison.