You can't use the c clips with the aero bars as they are alumnium and may deform (possibly).
Funny, I have had mine fitted like this for around 5 years now.
For me the C clips work better as I am constantly changing the set up from, 2 bikes, 1 bike and roof box, or just roof box.
I also think the T fittings are a bit less secure as it would take much to pop off the end cap of the aero bar and slide the whole bike rack off, with the c-clip, the feet of the roof bars stops this.
Just my preference. I also found with the t fittings and no rubber inserts I got terrible whistling on the motorway, unbearable in the wet.