If I were buying one now, I would get the Sporty's PJ2. It has headset jacks directly on the radio and the price is right. I don't know how good of a radio it is, but for a backup radio it would be nice not to have to fiddle with headset adapters in a dark cockpit.
You can't turn the pilot-controlled lights on with your cell phone, unless you can text or call someone who can turn them on for you. That gets less likely the farther from home you are. If you lose electrical power in your plane on a night cross-country, having a way to get the lights of some runway near you to turn on could make a big difference. Flying cross-country at night without a backup radio is a risk and, like all risks in flying, it's up to you to decide whether and to what extent you want to mitigate that risk.
Handheld radio capabilities seem to be more a function of the plane than the radio, in my experience. We have an ICOM A6 for the Cub. When we use it in the Cub, the radio's performance depends on which way you are facing. You may not be able to communicate with a plane that's in the pattern with you if it's in front of you, for example. But in the Champ, when I flew it to a Class D airport to get the radio repaired, I used the A6 and had no trouble communicating with Tower from the time I approached their airspace until the time I got to the tiedowns.
I did try using an external antenna for the radio. I found that it did not substantially improve the performance in the Cub, although I never did try mounting it vertically. The tedium of doing that wasn't worth it, so we are back to using the rubber-duck antenna that came with the radio.
