Welcome.
Based on no factual info, but overall sense from reading forums like this for years...Yamaha may have an edge in build quality/durability but Marantz is often preferred due to having Audyssey XT32 (automated room EQ system).
Both are well respected quality manufacturers and I would consider either a great choice.
Hopefully someone else will have knowledge of a feature based comparison.
However, you mentioned budget and Best Buy is seldom the best place for good value. I don't know how well you evaluated the B&Ws, but unless you are totally committed to them, I believe we can guide you to better sound at lower cost.
Let us know how flexible you are!
Also, I would delay replacing the AVR until you know why you want a new one. Yours should still have plenty of life. Certainly there have been some significant changes, but whether those changes will matter depends on your specific objectives. Maybe you have already analyzed this, but don't do it just because the BB sales guy recommended it!
You are right that there is no need to rush on this.
Among factors that determine sound quality, speakers are the biggest factor (along with room acoustics and the quality of the recording), so give speaker selection the most attention.