Just a guess but to me it looks like when compared to Fighter, the Slayer dual wielder has easier access to relevant style feats, some sneak dice, studied target and advanced rogue talent, for instance. Fighter on the other hand would wear heavier armor easier, have access to advanced fighter feats, such as armored bravery and then weapon specialization. In case of dual wielding, which I'm looking forward to trying this time on the MC, I'm more inclined to Slayer.
Mr. Maniloquent, in regards to 2h my MC had 10 Freebooter levels and stacked bastard sword dice. It gave access to Lead Blades and Bond/Bane that helped teammates also by buffing AB. If you want to wield big weapons it might be something worth looking into.