The case for a litre...
Balance shafts - 1000cc engines have them and therefore vibrate less than their 600-750cc cousins.
Grunt - we're talking circa 50bhp and 50ft.lb by just 4000rpm - enough for burbling around when you're not on it, making for more of a schizo bike - brisk but sane and smooth below 5-6k, mental above 10-12k, you twists your wrist and makes your choice.
Litre bikes are usually better equipped (to justify the expense) - slicker suspension, electronic steering dampers that back off at town speeds, hydraulic clutches, slipper clutches... genuinely useful stuff.
Bigger, heavier - better for living with on a long cross country commute or touring, maybe not quite so good for track but won't hold it back on the road.
Range - bizarrely they seem to use no more fuel than a 600 and often have bigger tanks.
Pillion-able - extra weight, stronger springy bits and a marginally bigger seat, and torque that makes smooth but reasonably progress easy.
Even so, test ride before choosing...