Ken, as you know I've thought long and hard about this...and we may already have discussed it in person, but for the benefit of the thread, here goes.
1) I already own a RIB...a Zodiac Yachtline 1 310. I also own two 9.9 HP outboards, one a stinky but fast Mercury 2-stroke from the late '90s and the other a 1985 Honda BF100 4-stroke in good working order (heavier but quieter and obviously cleaner and more fuel-efficient).
2) I had a davit failure in mere five-foot seas in Lake Ontario...with the engine running. Yes, that meant a four-foot extrusion flailing about in the water trying to pierce the half-descended RIB and snag the moving prop at the same time. This convinced me that davits are not appropriate in a seaway.
3) Getting even the empty RIB (120 lbs./50 kg.) off the foredeck in any kind of a wind has been difficult and potentially dangerous. So has lowering the motor to the transom, hooking it up, handing off a five-gallon tank, oars, crash box, bailer, etc. We decided it wasn't worth it. Were we to go exclusively to the Caribbean, I think it would be a 50/50 split decision, but then we'd be tempted to leave it on a painter. We would also have to retube our existing RIB with Hypalon, an approximately $2,600 cost, as it currently has nearly failed PVC pontoons (never get PVC...)
4) We've decided to go with a 10 or 12 foot Portabote, a 10 foot nesting dinghy with internal floatation, sails and oars, and a 2 to 3 HP 4-stroke motor with integral tank. We feel this gives us the best mix of capacity, resistance to damage, and the opportunity to have us running different errands in different tenders. Having a sail rig means our son will be able to use the dinghy for fun, and both boats together weigh about the same as the Zodiac alone. The dinghy will be lashed to the forepeak hatch, and will allow better visibility forward. The Portabote will go on the rails or the pilothouse roof, or even flat on the deck. The engine will weigh about 30-40 lbs./13-18 kgs. and my wife will be able to handle it solo. We will make up lifting bridles and use the spare halyards or even the topping lift and boom to get them aboard.
Being "humble" tenders, they may also prove to be less attractive to thieves, and their ease of handling and stowage will never tempt us to leave them in the water at night.
I may be wrong in my decisions here, and they aren't cheap ones, but I believe they represent a good compromise for long-term cruising/liveaboard lifestyles. Now, I have enjoyed bombing around on the plane in my RIB on calm nights in Lake Ontario, but the number of times I've carried serious cargo (does a case of beer count?) have been few indeed, and with the 1,000 lbs. my RIB can carry split between two tenders, I think we'll do OK.