I quite like the new LX C, and would probably give the seats a very slight edge. The IB seats are your more typical 2X2X2 layout (like old LX, or LH) but they are spaced enough (about 72") that there is room to go completely flat. The LX seats are a bit more modern, with a 2x2x1 layout. The difference it that they have very short pitch and you have to sleep with your feet in a little hole inside the seat in front. Both are comfortable and good for sleeping on.

The IB site used to give you the option to see all the different fares regardless of which one you chose when you did the search but you're right, that's gone now. I guess you'll either have to book two one-ways (I think the price is the same) or just call up Serviberia and make them figure it out for you.