Bill, what a great thread to start! Well, I'm not among the most experienced on this forum, but I'll add my experience with housing.
I started out in 2007 with an S&K barn on their Tri-Tel pole just to see if we would get martins, before I even found the PMCA. I don't know why I refuse to learn the lessons of my past.

After almost 3 weeks of fighting off the blue birds & tree swallows, I finally got them settled using Dan Drew's protocol.
When the martins finally laid claim to the house, I had issues with porch domination, so I only ended up with 1 pair that year. After dealing with the frustration of trying to lower the tri-tel pole by myself, etc. hubby bought me a Trendsetter-12 for Christmas!

Then I decided I also wanted to try gourds and eventually added 2 of the Deluxe PMCA gourd racks with 48 Troyer horizontal gourds. I modified the S&K & gave it away. So, I now offer a total of 60 cavities.
I'm stopping at that number.....for now....maybe ..... until I retire...maybe...
This was 2008 (before adding gourd racks) -
http://www.youtube.com/user/pmslums#p/a/u/1/RgwCSbK1T6I