Wifi vs. Bluetooth in HMI connectivity

When designing a user interface for a building automation appliance, often there arise a question should I provide a local UI at all as majority of my target customers already have a tablet or smart phone of some sort which can be used for the purpose?

If you decide that a non-local mobile UI will be provided, as only or as a supplement, number of new questions arise.  If I select Apps approach, which platforms should I support, and how many Apps for different platforms can I support? At the topic of the posting; should I select Wifi of Bluetooth as the technology for the connectivity in between the appliance and the UI terminal?

Traditionally I have favored Wifi, but recently I have considered that perhaps Bluetooth is better in some cases, especially from usability point of view. I have a personal example here:

In my car, I have an OBD2-Bluetooth dongle and in my smartphone I have an OBD application installed. Whenever I'm sitting in my car, it is very convenient just to take the phone from my pocket and launch the App simply by tabbing the icon, and then I have instant access to measurements and diagnostics of the vehicle. Very easy to do. After initial bluetooth pairing I do not need to do any other configurations when using the function.

Recently, I purchased an Android/iPad controlled wifi RC-car with live video streaming. At local retailer it costs only 30€. The toy has Wifi access point embedded. Yes, Wifi has bandwidth enough for live video stream. But on the other hand, the usability is not that good.

Prior launching the App of the car, first I have to manually change the Wifi network in order to have connection to the car. That's doable, but more inconvenient is that whenever I'm having the toy car powered up, my phone tends spontaneously connect to its access point instead of my home wifi. So, when I'm then supposed to do some internet surfing, I find myself having connection only to the car. And once again changing the network manually.

Wifi Camera Buggy from BeeWi

If I'd have a direct wireless access to my air conditioning, ventilation, heating, etc device, I'd definitely prefer just simply launching an App, instead of struggling with the network configuration first. Of course, there areissues like location of the connection point, regarding to signal strength and link distance, etc. which are to be taken into account when doing design choices.

Why not to use the Wifi of the appliance in device mode and connect to the existing Wifi infrastructure of the home? Well, that's one possible way of thinking. However, configuring wireless network when having wireless network connectivity only sounds like a shooting your own leg. I could only imagine how vendor of such product sinks into the flood of customer support request: "I tried to configure the network, and now I have lost connection to my device altogether. What I'm suppose to do now?"

I claim that from usability point of view, Bluetooth can be better option that wifi. The downside is that in generic case, Web UI can not be provided over Bluetooth, but a specific App is needed instead.