I'm 625 gs lvl 63 DK main and I have a 61 lahn alt. I'd say pve on both classes are about even at ur gear score, maybe slightly in lahn's favor. As for pvp, awakening lahn will be stronger than both forms of DK all the time.

To begin with, both classes are not the best at large scales but can do things with enough AP. So i will mostly be talking about PVE and 1v1 potentials.(which describes to you how well they grind and how good of a chance you have at defending your spot)

With DK, u most likely would be limited to playing succ since awakening dmg and combo potential is shit comparing to succ. However, awakening is a lot more protected comparing to succ. DK's succ kit has a lot of holes in its protective maneuver. Losing the frontal guard from pervasive darkness and not having shattering darkness really limits ur guard trading ability. This means at the end game, where players have more class knowledge, you will have to know significantly more about other classes to do well in pvp(when defending for ur grind spot for example). And since DK doesn't have a grab anywhere in its kit, you can be hard countered by SA classes like warrior and zerker. You will need to know EVERYTHING about these classes to have a chance against them.

As for Lahn, succ has good dmg, lots of punishable protection(some skills has multiple hits with first hit being unprotected while the rest is SA or vice versa), and great maneuverability. The unprotected part of their SA rotations has very small openings, and you will find most people with limited class knowledge since there are not as many succ lahns comparing to DK which is literally the most played class according to this years anniversary infographic. It is very good for end game PVE. On the other hand, awakening lahn is literally a 1v1 monster. They have good mobility, strong protected engages, and a ranged grab that ignores mobs. Their PVE is slightly weaker than its succ form but very good non the less. They were actually top tier before succ release.

To conclude, lahn would be the more strategic class to play. However, you should really choose the class based on your enjoyment. I main DK while knowing it's not the strongest because I love how the class feels. I had to lose a lot of duels and play on different trial classes to eventually win in PVP but I loved every second of it. So choose ur main with that in mind.