I think time is better, if you're riding for fitness. You get a better cardio workout by keeping your heart rate elevated for a certain amount of time. Your heart doesn't know how fast you're going on the bike. But if you can keep it elevated for a certain amount of time (I always hear 60 minutes at least) then distance doesn't really matter.

For me, though, it depends on if I'm making a loop route, or an out-back route. If a loop, then the loop is the distance and I usually go for speed and best time. If it's out-back then I might go as far as I want to go (like half the time available) and then come back. But then again if the out-back is more of a set distance (like the 7-mile length of the MUP) then I'll treat it the same as the loop route.