Caldwell, definitely, but I'm prejudiced.
EUL is a real hot bed of experimental builders of all types, particularly RVs and HRs. Actually, the whole region (Treasure Valley it's called) is chock full of homebuilding. I'm sure much of it has to do with ID being a very aviation friendly state, and with the back country at hand, considerable activity has to do with the great outdoors - Cub and Cessna territory and their homebuilt purpose-built brethren.
Caldwell is less busy than Nampa, with a longer, wider runway, less surrounding congestion. Hangars always seem more open than Nampa which seems more buttoned down. Nampa has a greater variety of hangars, from shade to large private. Most poor pilot accommodations are shade and open community hangars with no privacy and operated by the city. There are large private hangars. Caldwell's entirely entrepreneur, all privately owned, most individual, quite a few available for purchase or lease. This likely contributes to Caldwell's more neighborly atmosphere. Both have IFR approaches.
Depending on where you decide to live, there are other surrounding airports, but it means a significant commute if you live in Boise metro. Local urban sprawl is westward, so either Caldwell or Nampa serve well. Forget BOI unless you really like pavement, but it's not congested even though a Class C airport. Everything there is pricier, and the TSA makes it GA un-friendly and becoming more so.
If you want, send me a PM with voice contact info.
John Siebold
Boise, ID