Depends which playstyle you will find more fun for you.
your gear atm does not really fit any of the specs. you have very low spirit (below 6k) which more fits discipline, but you have also high haste/low crit, which perhaps does not go that well with discipline. you also gem haste.
as long as this is for lfr/flex, i wouldnt say that you need anything particular in your gear, just try out both healing specs and decide which of them you like most.
and if you decide discipline, then get rid of that renew glyph and dont use renew (there are exceptions, but just try to avoid it till you get more familiar with other tools)
Disc is mainly spirit until comfortable (at least under 12k) > crit > mastery
Holy depense on play style/raidsize. But spirit is always a main stat, until comfortable (i think most run between 12-16k). The more haste you take the more spirit you need. Its said in 10 man haste (soft cap or hardcap) > crit is better. But some choose to run mastery in 10 mans aswell. This depense how you heal.
25 man is mainly mastery > crit
You are also wearing exp gems in your staff, wich gives spell hit and shud be only used on shadow. But that might have been intended.
I recommend Icy veins and How to priest guides if you need more info about priest. They are really good.
sry should explained my gear a bit better =) the heirloom staff was on my main not to long ago. now replaced. the gems was when i tried out shadow. tho ofc am gonna change fix the gear when i decided spec. i do love disc but not sure if holy is better. as for mission? well it is a alt as you prob can see. my goal being to do flex/lfr and going normal when better gear=)
Disc is very strong considering it has nice heals, absorbs (avoids dmg) and does decent dps. Strong in both 10 and 25 but bit more on 10.
Tho holy is very strong in aoe department and can swich between chakras for strong single target, seen more in 25 mans, but also nice in 10.
In the end both are good if played well, just disc bring a bit more to the raid if you looking for absorbs and dps.