The Markins is a great head, I own a M20 the predecessor to the Q20. There is no comparison, a gimbal is far superior when it comes to big glass and since you intend to shoot eagles and I assume other wildlife in the area you will find a gimbal a pleasure to use. It is the right tool for the type of shooting you mention.
I chose the 5542 over the 5532 for portability. The shorter length makes it easier to place in the back seat or the trunk of the car especially with the gimbal attached.
As for which is more stable a 3 section or 4, that argument could go on for all eternity. With Gitzo’s manufacturing process, materials and with G-lock IMO both are equally stable. It would likely take a lab full of scientist and state of the art equipment to determine that both are relatively equal. IMO it is not worth worrying about, pick the one that best meets your requirements, for me it was the 4 section, for both my 3 and 5 series.
I was in Vancouver 4 years ago, beautiful city, it was the disembarkation port for an Alaskan cruise the wife and I took. We stayed on the waterfront and were just across from the big park with all the totem poles. I cannot remember the name of the park. I hope to get back there in the near future.