Hey there Purpleisbetter. I play a Holy Priest so my thoughts are Immediately Holy. However the truth is that both Spec's are really good. You should play whichever you prefer to play. Obviously come encounters might suit a certain Spec better then others. But I have Healed most Heroic Modes as Holy and haven't had any problems as yet which forces me to change Spec. Both are very capable tho.
if you dont mind, i went after your sig and found your armory (at least i think i did, crushridge?)
both specs are viable for heroic modes. i usually play discipline, and i can say that it seems very good for hc morchok, you can solo-heal one side.
i also like disc for slime boss hc, where you can preshield ppl before purple phase (get glyph of pw:s!) and barrier/ps can be very helpful on red/yellow/black combo.
for hagara holy seems much more powerful, can do hymns on electrical phases and speed up running via shields (but you will need 2/2 in body and soul for that, my suggestion is drop point in blessed resilience to get it)
for ultraxion i think both are about equal (i only played disc there), for zonz as well.
if you decide to play disc (on regular or sometimes) you may consider picking some output trinket for this spec, for example seal of the seven signs or windward heart, as with your spirit i think you could pass on tsunami proc as disc.