I haven't played much of 19 rogue, but I do have some insight to share on the matter. My reasoning for combat, is that I am not forced to wear daggers. I know that melee damage done while duel-wielding is a thing when you're assassination, and that it doesn't necessarily require daggers to trigger that passive. However, you will miss out on one of assassinations main damaging spells, mutilate. So it's not worth going assassination if you don't wear daggers. This may be a longer rant from my side, but bear with me.
So, since assassination, at least to my opinion, is "forced" to wear daggers (because you simply can't miss out on mutilate), means that you can't use shadowfangs. Now, this item is not just good because of the agility scaling. You get that from BiS assassination items as well. However, the raw damage on the weapon, and the proc is just so good for gimping. By raw damage, I do not mean dps. The dps is probably around the same on BiS weapons for both specs. By raw damage, I mean the actual damage of the weapon. Since ambush and eviscerate benefits from this, those abilities will do more damage. Maybe not significantly, but enough to make a difference overall combined with the previous factors, not to mention that you won't necessarily have much up-time on a target. This is due to how rogue is normally played, and the fact that rogues do not have any of their mobility spells yet - sprint, burst of speed/shadow step. So your up-time is definitely limited. Not to mention that they don't have a slow either yet (no crippling poison). With that being said, you want to do as much damage as possible in the short window you have during your up-time. Meaning, that a few swings with shadowfangs, which has more raw damage than daggers, plus potential procs, will be more beneficial since daggers will need a few more hits to even out the dps between the two. I hope that made sense.
Now, assassination benefits a lot from haste. However, you can make it work with versatility as well. If you do choose to go haste, you will miss out on the damage mitigation from versatility, which is a huge deal for rogue. Also, because rogue-friendly haste gear is pretty much non-existent, I wouldn't try the haste thing.
When it comes to talents, nightstalker vs subterfuge, you are making significant trade-offs. While subterfuge will have its uses with the double ambush, not to mention still being in stealth for little while when opening a target, you will be missing out on a lot of map movement. And let's be honest, rogue is an objective class that needs to be quick on its feet and be at the right place at the right time. The gimping style won't be much different, since with nightstalker, you will do 50% increased damage on ambush anyhow.
So when it comes to the trade-off between the two, you are simply giving up too much value for subterfuge.
Now, I will leave by saying that both specs are definitely viable. If assassination can stick to a target, they will do more overall damage. This makes assassination a viable mid-fighter in my opinion. Because you will have other classes to assist you, and make sure you keep your up-time to an optimal point. You will also be a force to be reckoned with, because the healers will forever be afraid of opening kicks. You will be able to sap back-lining healers that drops combat, or other high-priority targets that gets cc'd to drop combat. If you are a night elf, and have meld off cooldown while having an absorb on you, you can essentially do this while in combat yourself. And you will still be able to play the objective game to a fairly complimental level. However, people do choose combat, and nightstalker over assassination and subterfuge, and that could be due to some of the statements I made earlier in this post. Again, this is entirely my opinion on the matter.
Hope this shed some light on your question. Sorry for the rant, lol.
