You can gear in three possible ways currently.
1st best : Mastery/Versatility ( most instant damage, most damage from MB/MS ( two damaging spells you use by far the most, excluding the dots ), survivability )
2nd best : Haste/Versatility ( gives you a bit faster casts and a shorter MB cooldown, survivability )
3rd best : Mastery/Haste ( don't bother, versa too good )
Right now, the only casted spell you really need to cast is Vampiric Touch. Like, that's LITERALLY the only casted spell you REALLY need to cast in arena when it comes to your actual "I'M GOING FOR A KILL" rotation. Mind Flay is channeled, therefore you get value out of it even if you need to stop and fakecast mid-cast, therefore it's not a huge priority. Mind Spike is good obviously.
So if the only casted spell you need to cast is Vampiric Touch, and the haste gained from going for haste yields miniscule cd reduction on Mind Blast, it ends up being a bit of bonus damage on SWP VT DP and Mind Flay. SWP VT and Mindflay are a minor part of your arena damage ( obviously doesn't mean it's an IGNORABLE part of your damage ). That leaves us with DP, which doesn't really need any boost in its damage.
On the other hand, Mind Blast is the huge majority of your damage in arena, so it makes sense to gear to make the two instant ( basically instant since MS procs like crazy ) spells hit harder. It also makes sense to gear for Versatility, because its value is really high when stacked, and outperforms stats like haste, crit, multistrike, etc.
Also, when you're going Mastery, in the case you're not being trained/interrupted, doing the PVE rotation without the dot weaving part ( so only doing Spike Spike Blast Spike Spike Blast etc ) is higher damage than having VT SWP, using MB on CD and channeling Mind Flay into the target as a filler.
In the playstyle where majority of your damage is instant, it makes zero point to gear haste for a lower MB CD ( was it a 0.2 sec reduction or something when I've tested it ? Something along those lines, and not to mention that it actually loses value if you're being CC'd in any way or form, because for it to be truly effective, you'd have to be able to use it on cooldown ), and it makes zero point to gear for an obsolete playstyle ( VT SWP Mindflaying Mindblasting as a filler inbetween Horror cooldowns ).
TL;DR - neither gearing style is wrong, but if you want to get the most juice out of your character, gear Mastery/Versatility. If you're not being trained, and the enemies are presenting themselves to you ( a rarity ), whenever you're in the phase where you're between Horror cooldowns, off-dot the 2nd DPS and spam Blast on cooldown and Spikes otherwise into the 1st DPS for a higher effective damage than dotting that target as well and Mindflaying it; if you're being trained, you wouldn't get much value out of haste anyway, and this way your MB and instant MS hits way harder. ( if the DPS you want to Mindspike is in position of running away at any given time, it's better to dot it and Mindflay it )
I almost forgot to mention - if you plan on playing Shadowplay in any sort of form ( Shadowpriest/Affliction/healer ), Haste/Versatility is BY FAR the best setup combo to aim for, since you'll actually be casting 24/7 - you want DOTS up on all 3 targets, that's a significant GCD expenditure/20 sec. If you're playing Shatter, Mastery and Versa is the best.
Also, I noticed a familiar name pop up on the ladders some time ago : http://us.battle.net.../Talby/advanced
Mastery and Versatility are both equally good once Versatility reaches higher amounts of budget on gear ( IN TERMS OF DAMAGE ), therefore it becomes a preference at that point when it comes to enchanting ( more damage ? more survivability ? ), and the shoulder/pants dillemma in his case was him deciding he wanted to forego extra bit of stats that are actually useable ( haste on set pants as opposed to crit multistrike on shoulders ) in favor of more versa/mastery, but he's probably the best Shadowpriest to follow in terms of gearing if you don't want to be as anal and detailed as I am.
I've no idea how much time he spent figuring it out, but the same way that for Warlocks Wallirikz will be the best example of a properly figured out gear setup, Talbadar will be the best example for Shadowpriests ( if you already don't take my word for it ). That guy ALWAYS went for cheesy shit, I remember he had 15 different sets just for dueling alone.