I'd been thinking of writing a short blog on this for a while and hadn't got round to it. Headstock tuners are electronic (by 'electronic' I'm wondering if you mean pedal tuners?). They are all broadly similar, but performance does vary, I got a Snark SN-8 'super tight' not too long ago and it's better than previous ones I've had (guitarman, guitartech). By better I mean faster and more stable lock onto the note. Since Snarks seem to be about the same price as the bog-standard ones now I'd pick them over others.
The accuracy you get from one of these is about the same as what you'll get from a normal pedal tuner (the type you plug your guitar into), since they work basically the same way. One thing to do is position it where vibration on the headstock is strongest. They're cheap and quite convenient. However, I've been told by people who're used to playing with bands that they can be prone to picking up other people's instruments, this is one place the pedal tuner will win out, it just picks up your guitar, and is also maybe easier to use on stage if you have other pedals anyway (since you also get things like muting while you tune).
Newer pedal tuners like polytunes will also offer the ability to tune multiple strings at once, so you just strum and it tries to lock and show all six notes.
Finally, more accurate tuning. Stroboscopic tuners are more accurate than either headstock or normal pedal tuners and also more expensive. There are also 'digital stroboscopic tuners' which are a bit cheaper. I'm not sure they're really needed for everyday use, some people like them for the ability to do sweetened tunings and some studio and luthier work can need the accuracy. But these days a good tuner program for your computer or smartphone app can get as good accuracy provided you have a way to plug your guitar in. I like Lingot http://freecode.com/projects/lingot , there's a Windows version I haven't tried, http://lingot.gitara.org.pl/index.php.en .
What are the best headstock tuners (or whatever they are called)? And are they as good as electronic tuners?
Snarks seem to have the best reputation for the headstock tuners. My personal choice for a stage tuner is the Korg Pitchblack. Easy to see in the sun and dark, easy to recognize "in tune", plus you can adjust standard pitch away from ISO 16 A=440 Hz if needed. ( Depending on which "standard" is being used, A can vary from 415 to 466 Hz. The 1955 ISO 16 spec is currently the most commonly used)
Playing guitar and never playing for others is like studying medicine and never working in a clinic.
I think peterson makes one. If they do , it's the best.