Is it better to run your own node or use a public one? When using a pubic one does it matter which one you use? Also, with an S3+ do I need to change the diff?
p2pool was intended to be run locally, it is almost always better to run your own node.
If you want to try the pool out, try to find a node close to you.
Ours is US East coast:
http://minefast.coincadence.com