FP4 is considered more forgiving that Across because you are less likely to blow your highlights. Both are slow but as mentioned Across is king for its modern small grain and reciprocity characteristics. I also would consider using a faster film for windy days, dark days, slow lenses - so having two choices. Xtol is great but you have to mix batches and store it in air tight containers. Also it's not cheap. Consider Rodinal - cheap, lasts forever and mixed with water when you use it.