I would have to say breeches as I don’t really ride in tights as I do eventing and so I ride in breeches. I find they help me stay in the saddle more. Tights are great for dressage but even then I don’t wear them as much because I personally don’t like the feel of them.
Breeches because I find that tights can be too slippery in the saddle. I also find they move too much.
I prefer tights because I think they are more flexible and comfier. I also think that in the summer they are much cooler.