Tough call, my friend, tough call.
Truthfully, probably any of the three makers would be just fine.
I have seen some quality control issues with Peterson pipes - but, somehow they are just charming all the same to me. A "system" pipe is something you'll enjoy the rest of your life, and you can pick between a regular stem or a p-lip stem. I like the system pipe with a regular stem. These are nice looking pipes that are sturdy, and not super expensive. Like a favorite pair of broken in blue jeans.
All Savinellis I have ever had smoked great, down to the cheapest pipes they make. The variety of shapes and finishes is quite impressive. I like their semi churchwardens.
For me, Stanwell pipes are the most aesthetically appealing. I don't know about their new pipes since they switched to manufactoring in Italy, but the ones marked "Made In Denmark" have always smoked well and looked beautiful to me. Many of their shapes were designed by pipe making legends, and then machines were used to mass produce them for folks that couldn't afford a pipe from a master pipe maker/designer.
I think in the end, it's all about the same. Pick a pipe that just looks gorgeous to you, and you'll be happy. Before you light it up for the first time, see if it passes a pipe cleaner to your satisfaction, check if the drilling is to your satisfaction, etc. If something is blaringly wrong, any good pipe retailer will make it right, provided you havn't smoked it yet.
Whether to go for an estate pipe or not is a personal decision. I've bought both. Some people think it's weird to smoke something that another person has smoked and prefer new. If you learn how to clean and sanitize pipes yourself, or are willing to pay a few bucks for a professional to deep clean them for you, you can get a nice deal.
Enjoy the hunt.