1.) Optimally you want both specs (at least for 10 man), but overall I'd say that holy is slightly stronger right now (at least in 25, 10 is a lot more even with discs added dps having a much bigger impact).
2.) Disc until you reach an amazing gear level, atonement adds a lot of free dps. At a very high gear level you could probably get by with mostly dpsing as holy as well (and keeping up with the actual dpsers).
3.) Change them up based on the fight, but this is a decent build for most fights: http://www.wowhead.com/talent#pdH|
4.) Again, change it up if necessary, http://www.wowhead.com/talent#pXL|
You really should try both. Both are fun to play, my preference is holy but many enjoy disc, so you just need to try and see which healing style fits you better, reactive or proactive. Once you've made that decision, you can start mastering your spec of choice and raid with it. Whatever spec you choose, you will be viable for normal raiding in ToT. For HC I have no clue yet which spec will perform better. I suppose disc will shine in 10m with atonement helping beating enrage timers, where in 25m holy will shine with massive raw healing done.
That said, both specs are very balanced this patch. As a general principle, if you like Atonement (i.e. healing by doing DPS), then go disc. Otherwise, go holy.
As for talents, they depend heavily on the fight, but disc priests most often pick up Solace for regen, while holy priests pick up Mindbender. In tier 5, most disc priests pick up Power Infusion while holy priests pick up Divine Insight. Tier 6 depends on the fight, but it's usually a choice between Cascade and Divine Star.
1) Holy or Disc for Raids and why?
2) Holy or Disc for 5Mans and why?
3) Talents for Disc?
4) Talents for Holy?
1 - which one do you like more? why? cause game should be fun!
2 - which one do you like more? why? cause game should be fun!
3, 4 - play with them, find what they do and which ones you like most. why? cause game should be fun! just take mender
id answer differently if it was a matter of a world progress, on which i change/adjust talents/glyphs/spec each new fight and i guess it will be even more messy today with starting heroics.
but for 5man and lfr you really can do whatever you like. both specs are viable. talents are not as crucial in 5mans and lfr. play, test yourself, find what you like most.
I have at least one of every healing class and have played them all extensively for the past seven years and if you used to heal on a paladin, discipline spec on a priest is going to be much closer for you to get used to than holy is. Holy plays more like a resto druid than a holy paladin. Discipline plays more like a holy paladin.
I've been a little irritated with the changes to discipline this week basically making them nothing more than atonement spam if you want to get the best bang for your buck in raids, however, I do believe that Blizzard will change that and they still have a very full toolbox to use in other situations like 5-mans, PvP, etc.
Eventually you should try out both to see which one you like more but if you're just getting into it I'd say disc should be easier to pick up.