Not sure where it's at exactly right now, but a few weeks ago when I had the same question and came around the forums here was the information: Assassination is top spec for single target with OK AoE and nice burst and is the spec of choice. Outlaw is OK, but the RNG nature of Roll the Bones makes it unfavorable for some people, but it is king at dungeon content because it's AoE is amazing and it's burst is pretty good. Everything I've read says that Outlaw is competitive (not too far behind Assassination). Subtlety wasn't even on the radar, except for PvP.
So if your choice is already between Outlaw and Assassination, choose whichever one you like and you should be fine as far as fun factor, because unless you're a progression raider who absolutely needs to min/max Outlaw will get the job done just fine.
Granted things probably changed a bit in the last few weeks so this may not be 100% accurate. If it's not I'll let the more up to date folks provide the most current information, but based on how things usually go in Beta it's probably not that much different to make what I said completely irrelevant.
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Alexi
Unless you spend a ridiculous amount of time maxing them all out, it's a problem. And only the most seriously dedicated people who are willing to spend that kind of time won't have a problem with their artifacts all being at the same level. For the more casual crowd, it's more reasonable to assume that they'll have one artifact maxed (or close) and one a step or more behind, with their third (or fourth) being pretty far behind, which isn't really reasonable to use in a raid setting asking for min/max type stats and DPS.