Go to Spiti if it's first time as it will give you an idea on what you may get in Ladakh. However if you are doing it with friends who are experienced bikers, then you may try Ladakh. Ladakh biking demands a lot from bikers. Read some biker blogs to get an idea on what to expect in the trip.
Depends on the number of days and the route. If Rohtang is open and you have a month, you may do both. If not see the local weather and then decide. The rest decisions will be your's after you go through photos of these places.
Go for spiti valley brother... as its ur first time u will get experience that will make u strong so that next time u visit ladakh u can enjoy the travel rather than getting frustrated or mismangment from problems on travel