I would only ever ride the gag with double reins. Its the only way you will get the correct action from the bit that would make sense to your horse! Gags ridden with single reins are wrong and not how the bit is designed despite loads of people riding like this!! Sorry if this offends!!
Kimblewicks are designed for use with a single rein, different pressure/action to the gag.
What are you looking to achieve? Has the horse a low or high head carriage? fleshy lips? need curb pressure etc
Both bits have a purpose and work well when used and fitted correctly really depends on the horsey!!
I have a horse who I ride in a 3 ring f/l gag and it suits him and his needs, however jumping in double reins especially xc is seriously hard work at times!! We also have use of a small but powerful pony that is competed and hunts with my daughter in a kimblewick which suits the pony as he doesnt like jointed bits plus she has small hands and loathes double reins!!
They're barely comparable as they work in completely different ways and what will suit one horse won't suit another. Antifaz, for instance, absolutely will not tolerate a curb chain.
I agree with the posters above - they work in totally different ways and are used for different situations. I would also use 2 reins on a 3 ring gag. Because the bit is essentially a loose ring, use of the lower rein will cause the bit to lift in the mouth as well as putting pressure on the poll. This will cause the horse to lift his head. A kimblewick has a fixed mouthpiece and curb chain, so when the rein is applied the bit revolves in the mouth, applying some poll pressure and bringing the curb into action. This encourages the horse to lower his head and soften the jaw.
I've used both bits on very different horses. Jenny is ridden in a Myler kimblewick for jumping but she wouldn't tolerate a 3 ring gag and would probably just bounce on the spot if I used one.
Opinions on what exactly? THis is a really difficult question to answer as they're such different bits that you can't compare them.
I used a kimblewick successfully with my Welshy but didn't get on with a dutch gag for him. I needed to deal with a certain problem - one bit worked, the other didn't. You can't compare them, it's like comparing apples and oranges.