I disagree. Night stalker is by far the best leveling talent in the level 15 tier. Don't believe me? We'll see who gets out of that cave you can't mount in first. By the time you've traveled 300yd, I've traveled 360. By the time you get out of the cave, I've already mounted and have moved 80yd via mount--and that's with a nonepic land mount.
Shy away from sub for leveling unless you're really comfortable with it. Once things outlive your opening FW, you need to do some fancy work (compared to the other specs) to use your cooldowns, since vanishing will reset the mob entirely. Assassination should provide "comfortable" single target with ease, and combat has the best CDs/cleave, although leveling a rogue I had to avoid pulling >1 mob as often as possible (post-87/88; 85/86 were a breeze as they are on everything) - use what you have the weapons for. Don't forget how to sap, blind, and just roll that recup/SnD with deadly momentum. You may need to eat/bandage on occasion but it should be relatively rare.
May check out Shadowpanther's leveling guide to see what he has written for the 85-90 journey; honestly I found the dropoff around 88 to be harder on my rogue than anything else I've leveled yet.
If you're REALLY short on gear, don't forget either the AH or those gear vendors out in each area...
I'd just like to shudder at 88-90 memories. *Shudder* Really, really bad time and in such a hurry!
I played straight to get rfr90, and so by 88-89 I was in some kind of dream state. I was mutilate (which is contrary to my own advice), and I was of the impression at some point that by pressing envenom at just the right moment it would totally crit.
I generally recommend combat. One nice thing as you level is that your bandit's guile normally sticks around from mob to mob. You should have deadly momentum and a great uptime on recuperate.
Rogues are very hard to level, because they are so gear dependent.