Both stats have caps
As you likely already know, hit and expertise ratings have very discrete caps. Expertise's cap is 26 expertise, which equates to 781 rating at level 85. While a few races, like gnomes and orcs, have racial passives that boost their expertise, most races will simply remember that 781 is the cap in place. We won't see any real benefit to expertise rating over that number, and so keeping expertise under 781 is crucial to ensuring efficiency.
Due to the number of different mechanics that form a rogue's damage, we have a few different hit caps to worry about. Our special attacks reach their hit cap at 241 hit rating, which is considered to be the "yellow hit cap." The name refers to the fact that our special attack damage is displayed in yellow via combat text. The second hit cap concerns our poisons and is known as the "spell (or poison) hit cap." The cap for that is 1,127 rating and is the last cap you need to worry about. For reasons I'll detail later, the poison hit cap may or may not be important for your spec.
Finally, the last hit cap ensures that every single one of our white swings (auto-attacks) will land; it is set at 2,523 hit rating. That's nearly impossible to reach without specifically stacking tons of hit, so you probably don't have to worry about that for a very long time. Note that all of these caps assume you're talenting into 3/3 Precision, which every PvE rogue should have. Some players who already have great gear may only need 2/3 Precision, so I suggest looking at your current gear's hit rating and deciding what to do from there. PvP rogues have varying opinions on hit rating, but typically suggest at least 5% hit to ensure special attacks landing.
While hit and expertise have been perennial favorites in terms of stats, they obviously vary in value as we approach their caps. Due to the massive stat redesign that Cataclysm brought with it, agility is now the king of all stats. When choosing from the secondary stats, it's important to remember that hit and expertise may not always be the most valuable options. Based on your current gear, proximity to a particular cap, and your spec, you may have completely different stat priorities. Combat, for example, will always want to choose hit when compared with mastery rating, but that's only based on mastery's current value. However, haste rating is far more valuable for combat after the poison hit cap is reached. Keeping up with current class theorycrafting is going to be the best way for you to figure out what your individual stat priorities are.
Different stats for different specs
One of my favorite parts of the rogue class is the diversity of its specs. Blizzard's developers have done a lot of work toward giving each spec a unique theme. Because of the individuality of each spec, hit and expertise have different values for each spec. Assassination rogues deal primarily poison damage, which uses our spell hit chance values to calculate our chance to hit. Because of this, hit rating is incredibly valuable (compared to expertise) for assassination rogues. Expertise is less valuable for them due to the fact that even a dodged Envenom still grants the Envenom buff, meaning that the penalty for a dodged Envenom is fairly small.
Combat rogues value expertise rating much higher because so much of their damage comes from white attacks, which receive a huge benefit from expertise. Hit rating is still important, though, and in fact, the yellow hit cap and expertise caps are generally the first priority. Subtlety rogues also value expertise, as a dodged finisher can ruin their very complex rotation schedule. Because each spec values each stat in a different way, rogues of the varying specs will have different ideal gear sets.
Reforging and gems
The addition of reforging and jewelcrafting to WoW has given us the power to shape our gear as we desire. We can use gem sockets and the reforging vendors to rebalance our gear to ensure that we're always under or at our desired caps. If we need more hit rating, a gem can help us reach our goal. If we're over the expertise cap, we can reforge some of that expertise into a more valuable stat for our spec, like haste or mastery. Expertise gems are red, which conflict with our usual preference of choosing agility for all red gems. Hit rating is considered to be a "blue" stat, so you can use an a Glinting gem to both grab some hit and possibly score a socket bonus.
The goal is to remember that every stat's value is dependent on the value of the other stats we have access to, and we need to keep the various caps in mind when choosing our gear and gems.
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