I'll be backing Torval. I'm allied with a number of her factions. My opinion on slavery is that I've already made some nice money trading Imperial Slaves and doing her 'Freelance Employment Consultant' missions. She wants to increase the supply of Imperial Slaves, can't disagree with that. Along with Patreus, she also seems to represent the status quo in the Empire. Personally I think she'll back Arissa for the throne and not seek the throne herself. Harold's line is against slavery, so that won't be happening. Not sure what she'll do in regards to Patreus, as he could be either useful in an unholy alliance, else their rivalry could turn into civil war. But their interests seem to be mutual right now; Patreus needs a warhawk, Torval needs a conservative. Empress Arissa could be easily influenced by both and Chancellor Blaine.
EDIT: The outcome is this could definitely influence relations with the Federation in the next few months, especially if Patreus is victorious in someway and if President whatshername that's meant to be a James Callaghan of leaders so she can lose out to some crazy right-wing nutter, either through democratic means or through an English Civil War style purge of the Congress. We'll then have President Hudson in office, who will obviously be proponent of brinkmanship against the Empire. I honestly think that war will break out and combat zones between the Empire and Federation will spring up before the end of the year.