Two reins - makes the aids more refined and is kinder to the horse as you only need to use the curb rein when needed and it isn't being used all of the time.
Double reins are the best, as you can give precise aids - i.e ride off the snaffle rein and employ the curb when required
However than said, not a sin to use roundings, but means the curb is always on, so can be too much. Split reins are slightly better IMHO, but not many people use them
I think the temptation is to put roundings on as people are scared of two reins, however its a good skill to learn. I can manage three reins, whip and polo mallet, it really isn't that hard!!
Its OK guys - I know how to ride with two reins - been a while as my first pony was the one I hunted in a double bridle but it should come back to me....
I just need to sort out this nappy mare and thought one rein might be easier to handle in a fight....
She tried very had to piss off with me down the drive on Sunday so I need to teach her a lesson - don't want her going up though so think the double reins might be better as I can drop the curb in an emergency...