Definitely preference, and agreed that hiking will mean you need more time, especially in Glacier.
I'd allow a minimum of 4 full days in Yellowstone, one of which should be spent visiting Grand Teton National Park, just to the south. I'd suggest 5, and add the Beartooth Highway (northeast entrance of the park) - it would be a dandy on a motorcycle! However, for that, you'd need to be sure to be there between mid-July and early September, or the road could close due to snow. If you need hookups, you will need to camp at Fishing Bridge, or outside the park (West Yellowstone being the best option for that, with several decent choices of CGs with hookups). If you can go for 5 days with no hookups, then almost any of the other reservable campgrounds inside the park will work for you, assuming your rig will fit in the site. Campgrounds in Yellowstone are older and many sites won't accommodate much bigger than about 25-28' total rig length.
In no case would I ever recommend spending just one day in Yellowstone. :)
Can't comment in detail on Glacier, as we have not yet been there. Going later this summer, if plans hold.
