Either sound be fine. You might want to check the 12.3 EX RN to see what changes were introduced between R11 and R12 rto see if any might affect you. I believe between R11 and R12, all the changes are PR bug fixes. Support does not update that recommended page very often, maybe once or twice a year. I think at last update, R12 was too new for them to consider it. Please support recommended releases are based 100% on stability, with no regard for any feature support.
I think at this time the currect thinking on these [now] older platforms is to go with 12.3R[latest]. At the same time, if you are in no major rush or if you do not have an upcoming soon to happen maintenance window, I might suggest you wait for 12.3R13. 12.3R13 is suppose to be out very soon, and would then become 12.3R[latest].
For these older platforms, which all have or will soon have newer replacement platforms, I would recommend staying the course with 12.3, and not looking at any of the other releases, again specifically for these EX platforms. I did see a note regarding 12.3 in that after R13 release, there is no plans for another R release until very late in 2016. So you would not expect to see a 12.3R14 for 8-9 months or so. If any major bugs are found, most should already have been discovered, Juniper will but out a S release based upon 12.3R13.
Hope this helps.