You get a larger increase on stats from upgrading chest/helm/pants than you do from upgrading shoulders/gloves, so you should upgrade one of the latter. Which one depends on which piece carries the stats you value more for your spec.
For example, being main-spec disc I chose to upgrade my shadow tier chest first since it provided a boost in haste and crit, whereas with any of the four holy tier pieces one of the stats I'd have been elevating would include spirit, which is of relatively lesser value. So upgrading the chest gave me the biggest performance increase (and also benefitted my shadow off-spec as a perk). When I had the opportunity to later upgrade one of the main set pieces, I chose to upgrade the helm first since my haste and crit were, imo, well-balanced and because.... it looked better. But shoulders and gloves should definitely come last, regardless.
Bigslick of Lethal, Thunderhorn-US
HM: 7/7 M | BRF 8/10M
Tues/Thurs 7-11pm CT "Those who deny respect for others deserve it not for themselves."
I suggest you get the robes from 25HC princes instead of the t10 chest and stick with head+gloves+legs+head for 4 set if you're holy...
Admittedly while legs/head seem the most logical choice you should probably see what gives you the highest stat increase ( and I don't mean stamina :P ).