hy antoni hsk is better having also the flange contact, but be aware, that sometimes the vendors, even if they allow for option between bt and hsk, they are not paired, in the sense that it may be possible that the biggest bt has no hsk equivalent, while one size smaller bt has hsk equivalent
i would definetly look on the exact sizes of bt & hsk that they offer
the rigidity of the hsk is better than bt for small tapers, but at bt50, they are quite alike, and after that, on the theoretical curve, bt has the upper hand; on the long run, hsk toolholders may cost you more
bt50s are not for high speed machining, at least they are not to be reved at high rpms such as smaller bt's
as for torque, if there are different spindles, choose the one that has greater torque at common rpms; depends what the vendor offers; checking the link that you provided, i would ask for the spindle diagram for each unit, and available interfaces for each ( also 1st spindle type seems to be reasonable, 2nd may cost you more in the long term, while 3rd is for hard materials ) / kindly