I don't know if most people feel this way, but I don't like to leave a martingale collar on my dogs at all times. They are in their crates a lot, and I've heard horror stories about collars/tags getting stuck and well you get the idea.
My dogs don't wear any collars in the house, and they get their regular collars put on if we go on a trip or go out, and they get their martingale collars put on if we are taking them for a walk.
But to answer your original question, I'd probably go with something cheap for now. If you want something cuter, you can try Lupine collars. I think they make martingales, and they have a lifetime guarantee--even if your dog chews it!