Both are good atomizers, nowadays used mainly for dripping.
They use the same threading.
They need different drip tips though, because they are constructed differently.
510 use the standard drip tips (which were originally designed for this atomizer),
306 use special, longer, barrel-type drip-tips.
I would go with the 510s, just because they are more widespread and you can use
your drip tips for the rest of your devices.
If dripping is for you, you might want to check out the 306es at a later point anyway though.
They have their fans, preferring them vape-quality-wise over the 510s.