Surprised so few people have an opinion on the topic.
I threw together a comparison chart of Darklight vs. Darksteel, with the melds I intend to go with, no forbidden melds are included, as I'm not 100% on caps, but I will of course be forbidden melding. (Melds = Purple)
Code:
Set DEF STR VIT DEX ACC PRY DET SPD CRT
Base Stats 0 203 264 190 341 359 202 341 341
Darksteel Tanking Gear 733 356 424 290 461 427 256 432 371
Darklight Tanking Gear 733 312 432 190 453 527 212 371 401
I lose 8 VIT (which I can probably forbidden meld in somewhere, but I'll have to check caps) and 100 parry (about 2.8% parry). However, I gain 100 DEX which = +8.6% block rate, and the additional 44 STR puts me over the next 2 thresholds increasing damage blocked by 2%. I also sacrifice 30 crit, but gain 50 skill speed and 44 determination.
I think the 13% increase in mitigation from DEX alone is worth the 2.8% parry rate sacrifice. The other stats should lead to significantly higher overall damage with a slightly reduced crit rate. Sacrificing the 8 VIT if I can't manage to meld any more VIT, it's tough but it shouldn't be a deal breaker.
The crafted set seems to be the clear winner, relying on an itemization gimmick, but still. A nice added bonus is all these accessories will work great for any physical DPS as well. Although you might get marginally less DPS performance out of them compared to darklight, you'll gain a lot of extra HP.