As mentioned, it's probably not the best idea to skip the collection quests in Northrend. Thus, you will end up doing most of the quests anyway. You will definitely not run out of quests before you hit 80 if you don't skip anything.

Reason not to go? Hmmm, well you will have more quests at 80 for gold. Can skip collection quests in Outland. This might work out to be faster to 80, if that is important to you (but, you I don't think you will not get as much quest XP in Outlands, so maybe it wouldn't be faster, I don't know). Regardless, getting from 68 to 70 will take no time in Outland as well.

Personally I had difficulty with the normal Northrend instances. I was a fresh 70 when I went, so I wasn't geared to the max like a lot of folks. So, I actually decided to skip them for the most part until I hit 80. Although, I do need more experience with multiboxing instances, you may be skilled at it so it might work for you.

The daiiles in Northrend (at least the few I did) do give XP when you are not 80.

So, if you're excite to go and see the new zones, then go. Otherwise, leveling to 70 or higher in Outland won't be a mistake. Either way it won't make much difference IMO.