I went with a 561 for a (slightly) lower profile, easier mounting of bike onto rack, and the bike feels more secure. (ive known of clamping arms shift and the bike falling over while still tied on by the wheels)
Disadvantage being if its a 20mm bolt there is no security as far as leaving the bike unattended, but i wouldnt leave any rack unattended myself.
my bike faces backwards, so the bottom of the steerer tube (under the fork crown) whistles from time to time on the motorway, which can get a shade irritating, but its by no means all the time. I reckon a 591 would still do it.
TBH if i was going to go again, i would consider a rear mounted rack i think to keep the bikes out the wind (slows the car down a surprising amount), but i had the roof bars already, have no tow hitch, and the upper lip of my boot is plastic trim, so it would have cost loads more for what is occasional use.
Ikea bags for front wheels in the boot.