Plus bike will give more confort and grip when things get really rough vs a regular 27.5 or 29er bike. Beginner and less experience riders will find confidence faster on those tire. This is relevant mostly for mtbiking in technical single track. Experience mountain biker will often (not always) find these bike slower and sluggish for agressive riding (deformation of larger tire in cornering) For the type of riding that you described, they're probably not the best tool... That said, even if I own a FS carbon 29er (140/130mm travel), I'm still regularly riding my rigid aluminium fat bike with 4.7 tire in technical single track, flow trail, bike path or around the neighborhood with my kids because bigger tires are just fun. So even though a road bike with 650b, bigger 700c or a hardtail 29er should be a better choice, if you feel like buying mtb+ it may open some new riding too.
My brother has a FS fat bike with 29+ (3" Maxxis Chronicle) for summer riding. We exanged bike this week on a 30km single track ride and I was really impressed. If you'r a tall guy, 29+ really keep their momentum while rolling and just eat everything you throw at them...As for suggestion, look at RSD bike offering. They have 27,5+, 29+ and 700c with 45c clearance with pretty reasonable pricing (like 2300$us for a complete 27.5+). Also Salsa have both carbon and aluminium 27.5+ bikes. Something like a Salsa Fargo should be on your list in my opinion but it's not a mountain bike!
Thanks - very helpful advice. I like the "fun" part. I'm not likely to get very serious about off roading. Also plan to do some riding around with my 6 year old grandson on much less challenging stuff. A plus bike - especially 27.5+ is very appealing. I also noticed that Salsa makes a convertible model that can be run with either 27.5+ or 29er wheels. Not sure how it works -- but apparently there is a switch in the suspension system that changes the bike geometry to accommodate either wheel size. Every time I see someone on a real fat bike, they seem to have a big smile on their face, though! This may sound like blasphemy, but I've even seen some 27.5+ e-bikes. Hey - why not? At 74 orbits around the sun, I don't have to prove anything anymore.