Damage wise, /Kin will always beat /Dark except for very specific scenarios (eg: AV fights) where you're less likely to take full advantage of Fulcrum Shift. While with /Dark you'll get a consistently good - and more relaxing - performance at all times.
Having played all three combos here's my quick rundown: Fire/Dark
Great all rounder for both solo and teams at any level. It's the most survivable toon by a long shot, while still dishing out good damage. Fade is a great team buff and also helps to keep your squishy Imps alive.
Fire/Kin
There's a reason why this was the go-to farmer for a long time back then. If built right, it's an insatiable killing machine. But, again, it's a late bloomer and requires a bit of investment before it shines. Turns everyone in your team into a wrecking ball. Imps are likely die a bunch, unless you immediately control the mobs or spend more time than you should caring about placement or throwing in extra heals. Fire/Cold
Late bloomer, but very powerful - especially against AV/GMs or on fast pacing teams. Bonfire and Sleet give you a pretty decent control of the battle field which *almost* makes up for the lack of self defenses (except for arctic fog) or heals when leveling.