There really is not much of a debate here when it comes to the T9 vs T10 for the average hunter. The T9 2 set bonus makes it "usually" a no brainer unless you are the type of hunter that never actually looked at what the gear says or never reads any forums or blogs.
The basic idea breakdown is this.
Keep the T9 2 piece bonus as long as you can, if it is the 245 version, then keep it longer and if it is the 258 version you might need to consider keeping it until you get the 4 277 T10 versions.
However, there is another angle to look at, and this is what I am judging it on. If you are still gemming agility keep the T9 2 piece for the bonus. If you have moved along to the gemming for Armor Penetration then you can switch to the 4 piece T10 sooner then you might otherwise. (With the deathwhisper 25 legs or the crafted legs)
I already have the 4 pieces of T10. Helm, Shoulders, Chest and Hands. All 251 as I have not gotten lucky enough to win a mark to upgrade them or win the 264s in VoA 25. I always lose the rolls. Bad luck sucks sometimes.
I considered buying the craftable legs, but being a slightly better version drops off of lady deathwhisper 25 and that is an easy encounter I will just wait it out and hope they drop and I can win a roll for them. Unless I keep winning the saronites while rolling. Seems that is all I can win. Then I might get the craftable legs while I am waiting. Just not paying a lot for them like I did with the boots. Boots where worth it.
I am still rocking the T9 set for the 2 piece bonus with the T10s for the 2 piece bonus. Once, or should I say if, I get the deathwhisper legs I will change to the 4 piece T10 and finally drop my beloved T9 2 set bonus. That is because, while still at 251, and T9 would make more sense, I am going to be switching all my gemming to ArP instead of agility when that time comes. And now the 4 piece T10 will make a lot more sense then the 2 and 2 I have been running. Mind you, my 2 pieces are 232, so that makes my decision a lot easier. If they were 245 or 258 I might be seeing things a lot differently.
Sometimes while looking at the gear deciding on what to aim for as an upgrade I wonder why blizzard decided to give such a downgrade to the bonus on the gear. I am still new to the game. T10 is my first tier gear. Sure, I have some T9, but that was already out when I hit 80 so to me it might as well have always been there.
I have been thinking about working on offset things for a while until I get the marks. Perhaps keeping the T10 Head and Shoulders and going with the frost Chest and Hands and deathwisper legs. I have not typed it into any engines yet but I plan to. Somehow the T10 bonus seems so lackluster to me that offset for "gear score", excuse the language, seems like a reasonable idea.
No matter how much I read around I can never get a clear answer on any of the issues with T10 vs T9 in the ICC era. Maybe that is a good thing. I read a lot of people saying what is the best. The best if you are marks, the best if you are survival, the best if you gem ArP as marks, the best if you get this drop, the best if you get that drop, the best if you have this skill, the best if you have that skill. You get the idea. So many "ifs". Somehow it was so much nicer when you said, this is the best and that was that.
One thing I am learning more and more as I raid with more people is that it isn't always just gear and skill that make the difference. Two people can have the same gear and the same skill but they play with different styles. It might get my flamed to say it(because people will say that style is skill), but I really think that play style comes into the equation when choose gear now. Being each choice includes an "if" with it... if you have this... if you spec that... if you do the other... but there is my if. If you give two people the same gear, the same skill, the same knowledge and let them loose on a boss you will see two totally different results. No, it will not only be the RNG that makes the difference, it will be the style in which they play.
How do you factor that into which gear is best or you now that there are no stand out things that are must have no matter what?