I vote for the BX I've found the Xantia is better built, with less plastic, and also less noisy. However, it got too heavy! Too heavy for the available engines, any way.
Have droven the BX 16V and the Xantia HDi. Sold both for reliability problems, but the BX 16V provided an incomparably better driving experience.
Aceleration on the BX16V was very good, not too far from performance cars.