The Blu-Ray standard does not support 50p, so you can only export in 50i. Regarding how good/bad 50i is going to be compared to the 50p, it depends on the kind of the de-interlacing algorithm your TV uses. Cheap TVs use less good algorithms, for example. But no matter what, 50p is always superior.

I expect the BD standard to be updated with 50p and 60p in the next 1-2 years, along 4k and more 3D modes. But that will require a new BD player too.