One of the most common questions on the WoW hunter forum is "I have items X and Y and I can't choose between them, which should I use?" Answering these questions for yourself isn't too difficult, and in my opinion is part of the fun of being a raider (yes, I'm a nerd), so I thought I'd step through it here. Maybe if a couple people read this, they'll have a little more fun looking through the upgrades for their own hunters. I'll use a real question from the forum's front page today for my example, since it's a pretty good one: Marrowstrike vs. Twisted Visage.
To start with, here are the two items in Wowhead's comparison tool. Right at the very top of the list you'll see that TV is an item level 232 item, while Marrowstrike is stuck at 219. At the very least, this means that if they had the exact same stats, TV would be the better item, since it would just have more of everything.
Unfortunately for the confused hunter out there, they're not itemized identically at all: TV has 70 hit and no sockets, while Marrow has no hit, 60 armor penetration, and a yellow socket with a 4 agility bonus. And this is where most people are going to run into trouble. They're going to start thinking about what gem they would put in that socket, if they'll have the bonus or not, how much does the reduction in crit from TV to Marrow cost them, and how much the gain in armor penetration is worth. Most of the time, doing all that may feel like the right thing to do, but it's just creating more of a problem for yourself than you need it to be.
Remember: more item levels on TV means that it just innately has more possible damage per second contained in its frame than Marrow, which means you only need to decide if you can make use of the majority of those item levels. So what you really need to ask yourself is this:
"With my curent gear, would TV put me way over the hitcap of 263 hit rating?"
If the answer is yes, then you're better off with Marrowstrike.
If the answer is no, then you're better off with TV.
Obviously this example isn't universal - sometimes you're comparing items with identical item levels - but the principle is going to remain the same. Reduce the complexity of the question you're answering and the question becomes much easier to answer. Most of the time, gear comparison between two pieces can be narrowed down to a single stat, or between comparing equivalent values of two different stats.
Take a look, for instance, at Hellion Glaive vs. Twin's Pact. The two weapons are very nearly identical, but the staff has 90 haste rating and the polearm has 90 critical strike rating. Everything a hunter can do can crit for more damage, but only auto shot and steady shot can benefit at all from haste. Therefore, you should always prefer critical strike rating to haste rating if at all possible.
"But wait!" the astute reader may say, "what if I put your two questions together? What if I've got two items where one has haste and a higher item level, but the other has crit and a lower item level?"
Your question brings me to the last comparison of the day: Twin's Pact vs. Lotrafen, Spear of the Damned.
This one is near and dear to my heart, as I'm currently carting around a Lotrafen and it seems quite possible that I could see the staff drop before Anub drops his polearm for me. Assuming a feral doesn't want the staff (I'll come back to the melee vs. ranged thing at the end of this), then it seems like a tough comparison. Really, though, we're just looking at another instance of that first comparison. The extra item levels on the staff really show: before you even count the socket, it has more agility, more armor pen, more stam, and more AP. More of everything just on its own, and it'll be even better once there's a socket in it. Sure crit rating is better than haste rating, but even there the staff has three more than the spear. A marksmanship hunter especially gets a lot of his or her damage from auto shot and steady shot: two things that benefit from haste. Considered along with the superiority of the staff in every other way, you would be doing your raid a disservice if you gave yourself loot tunnel vision and let the staff go to waste.
I almost did that once myself: when TV dropped the second time and our feral already had it, I didn't send a bid to the master looter because I really, really wanted Lotrafen. But the reality was that TV was a much better item than the Relentless Edge I had been using, and it would have been foolish to let the staff get melted. The ML rightly charged me a single point and looted the staff to me.
You'll notice, though, that in the case of both staves I mentioned feral druids. This caveat includes all the melee classes, really, it's just that ferals are the only ones that can use staves. There's this important stat on melee weapons that a lot of hunters seem to ignore, and that's the damage the weapon actually does when you hit something with it. Remember how, in the previous examples, the better item was the one with the fewest wasted item levels? Ok.
A hunter wastes every single item level spent on melee weapon damage.
You are hurting your own raid if you take a melee weapon away from a melee class if the weapon's actual DPS is an upgrade over the one they're carrying. Conversely, they're hurting the raid if they take a ranged weapon from you if it does more damage than your current one. The only time you should take a melee weapon over a melee class is if they already have a weapon with equivalent damage. The first time Lotrafen dropped for my guild, the feral and I both rolled for it because we were both DKP capped and we were both using Twisted Visage. The weapon was a very similar upgrade for both of us. If he'd been using a 212 DPS weapon, I would have passed to him without hesitation (in fact, the bid I sent to the master looter was actually "20 points, pass to melee").
So there you have it. If you can't decide if something is an upgrade or not, look for how you can simplify the question and remember: always consider the good of the raid first.