I haven't leveled DRK enough to definitively speak on it, like some have done so. Theory doesn't always equal understanding.
However, I would like to clear up some of the common misconceptions about DRG..
The wyvern: Always underestimated. Many times I've been in a party on something other than DRG, and laughed at my good fortune that I'm outparsing the DRG on something weak to piercing. And then I'd look, and it turns out.. I was doing something like 28% of the overall damage, the DRG was doing 23%, and his wyvern was managing 8-9%..
Any self-respecting DRG knows that Deep Breathing is important for soloing, but it isn't anything to laugh at for damage, either. Merited down to 5min, with Wyrm Armet, my wyvern's cap is usually 510 on a breath on colibri. Without it, it's still about 60% of the time landing unresisted for 255, about 20% of the time landing a partial resist, which tends to be closer to 75-150, and 20% being a full resist for 5-25ish.. The lower bound is obviously not that flattering, but when I see a SAM gloating because he managed 200 more on a Penta Thrust on birds than I did, but he didn't take the wyvern into account, I can't help but chuckle.
Damage output isn't the be-all, end-all of a DD: Now I know this concept might be met with criticism.. I'm basically telling so many DDs that their e-peen won't actually win them chicks. Yes, a DRK can do some staggering numbers. Yes, in end-game, a DRK is likely to be able to outdamage a DRG with the same amoutn of effort/gil into their gear. The DRK might even manage to do fairly well in merits vs. the DRG. So, what does the DRG have going for it? Well, a good DRG knows it isn't enough to be able to do a ton of damage, but to be able to do the most damage possible and still not piss off your healer until they let you die out of exasperation.
Why? Again, the wyvern helps. A DRG's overall damage output is split. This means the DRG isn't accruing the same enmity for DoT as the DRK. Of course, sometimes, the DRG still does, but that is more often due to piercing weakness and variances in gear/knowledge of job. Then, the DRG has High Jump and Super Jump to help mitigate some of his enmity spikes. No, he can't spam the jumps constantly, so obviously, he is still capable of taking hate, but he'll do so less often, save the healer some hassle, and help the party run a bit more smoothly.
Instead of popping a cluster of moves that requires a healer to completely focus on the DRK for a time to keep him from killing himself (Often enough, before the mob dies).
At 75, the DRG can contribute in another way: Angon. As has already been mentioned, Angon has its uses in end-game for reducing def for an enemy, and benefiting everyone. The DRK has the zerg strat, if either he can afford it, or commits the time to gear for it, but in and of itself, the DRK doesn't contribute anything beyond that (and a single DRK can't zerg, so you need several of them to make use of it). One DRG can keep a 20% def down effect for 1.5min without hurting other merit options, and a second can then allow it to be kept up indefinitely.
Versatility: Well, DRK has tons. That is, of course, if you like hitting things until they kill you or you kill them. Don't forget, you can drain and absorb. But the tool that kills it hardly makes the difference. The DRG can DD, can heal a balanced party, can solo, etc..
Not that FFXI players allow diversity or trying something new, but DRG could easily change his playstyle to fit each party he goes to. He can be a kinetic healer, he can straight DD, he can say screw the party in general (I soloed from 60 to around 67 just because I was annoyed with people at the time, and it only took me a weekend plus a couple of days).
The DRK is completely reliant on the party. Sure, there's campaign, but that is available to anything. /DNC opens things up a little bit more, but a DRK/DNC is never going to be able to bring the majority of its abilities to bear well..
Gear: This is where I've always felt bad for DRG.. Drachen armor is a plate mail that has the defense of pudding, and is competely inconsistent with the rest of a DRG's gear. DRG gets locked out of things deemed to heavy for it, and then watches a Beastmaster strut by with it on (whose AF is hide and furs). There are a lot of amazing options open to DRK that are closed to DRG (Luisant, Haubergeon, Hauberk).
Top end, sure, it begins to even out. Many have brought up Hecatomb for both DRK and DRG, and that makes sense. I would, however, consider Hecatomb Harness the second best body for DRG, with Zahak's Mail being the top WS piece (As it influences Penta Thrust damage directly, and crit rate for Drakesbane, and the 2 str lost might knock the fStr down a notch, but the 10 dex is definitely going to improve your crit rate, not to mention the actual crit rate on it). Yes, Hecatomb's 10 acc is a huge boost in the direction of DRG's multi-hit, but the dex will make up some of the difference, and it really becomes a question of "Can I sacrifice the acc for the damage, crit, etc." which is dependent on the rest of you gear.
Otherwise, DRG has about the same difficulty as DRK in a number of ways to get perfect gear. In my opinion, in which gil has been easier to come by than specific items, as I can farm, BCNM, sell myself, etc. to make gil to buy something, DRK has a benefit in having a better variety of AH-available items than DRG. DRG, however, for those who are the diy type, has a number of viable pieces available through NMs and events that aren't terribly difficult to acquire.
In the end, though.. This is a terribly long post giving my opinion of preference of DRG over DRK. Even when I haven't been able to beat every DD in the party, and my e-peen hasn't been indulged, I've still had fun with DRG. It was my first 75, and it's always been great to be able to solo the majority of things needed for other jobs, instead of pester friends for help.
Truthfully, though, I haven't experienced DRK, so I'd be a bit of a jackass to say what fun it might or might not have been..
They're both viable jobs, they're both DDs.. Don't listen to all of the bs of the "lolxxx" .. FFXI is full of a bunch of closed-minded people who thinks that actually makes some kind of a point. I tried to give you an honest accounting of DRG, which was my preference. Others have stated their preference of DRK, so simply compare, and choose for yourself.