I think you have 3 options (really have about 10, but I'm sticking with these 3).
(1) A 36" ATI 6 or 8 bulb T5 fixture- pros: proven light, lower up front cost
(2) Two radion XR15pros. Pros: what you are currently using and know you like, no acclimation worries having to move frags from T5 only to LED display later (but I have no idea if this is a big deal or not to be honest). Negatives- going to be more up front cost
(3) Hybrid- A 36 inch aquatic life fixture with one Radion Xr15pro in the middle. Mount the fixture at a height that you need to get the spread you need from the LED, or focus your high light corals more directly underneath the LED. Pros / Cons: cost will be a little less than option #2 but more than option #1. You will have a great combination of LED and T5. Two proven winners.
Another small thing to think about. Do you want to have all your frags on the bottom on racks, or do you want to use tiers / multiple heights? With option 1, your par will be very even throughout the tank. Since you said you are going to do SPS, LPS, and softies, this might be a small issue to consider. With option 3, you would have high to low par zones created which you will easily be able to place your mixed reef frags with your softies and LPS being on the outside areas and your higher light corals in the middle directly under the LED. I'm sure someone else with more knowledge and hands on experience on lighting mixed frag tanks could comment on this and share better insight. I'm just thinking out loud wondering what I'd do or want. Lol.
