I think before you get too worried about "battery drain" you need to just try it and see, unless you're driving for hours and hours every day I find it hard to believe it'll make much difference. Even then, it's a small price trade-off for being able to see track names and control the device using the car's own controls rather than awkwardly (and potentially illegally) having to interact with the device itself.
I used a regular AUX cable in my old car and there's no chance I'd go back to not being able to control it all via the car itself, it's archaic.