If you think you're going to accumulate towards a travel package, I'd stick with rewards points, especially if you're elite. 10, 12 or 15 points per dollar is better than even triple miles if you think you can get to a travel package. My math might be off, but this is the way I think about it:
A travel package for the category 1 to 5 cert gives you 1.2 miles for each 2.7 marriott rewards points (e.g., 120k miles for 270k marriott points). Even if you don't want to use the 7-day cert, if you're going for a travel package you should only forego marriott rewards points if you can do better than a ratio of 1.2/2.7 -- or .444 -- that is, even if you're willing to discount entirely the 7-day cert, you'd want to earn at least .444 x the amount of marriot rewards points you could earn for the same stay.
So, if you earn 10 points per dollar (as a non-elite), you'd want to earn at least 4.4 miles per dollar before choosing miles. If you earn 12 points per dollar (silver), you'd want to earn at least 5.2 miles per dollar before choosing miles. Etc. So, 3 miles per dollar just isn't good enough for me, if I'm working toward a travel package. Also, you have to take into account that you only get to 3x miles starting on your second stay -- so that further reduces the value of the promo. For example, if you have 4 stays during the promo period where you'll spend $250 each, you will only get 1x miles for the first stay and 3x for the next 3. So, that's 2500 miles total, for a $1000 spend, for an effective total of 2.5 miles per dollar. Even as a non-elite you could have earned 10,000 marriot rewards points. The more stays you have planned in the promo period, the more the effect of the 1x miles for the first stay is reduced.
Even with a 3x miles promo, I personally would only choose miles if (1) I had an immediate need for a mileage reward and was close, (2) there was some other special promo. For example, I recently changed my earning preference at marriott to miles for 2 "hits" on the USAirways "grand slam" promo, because they were relatively inexpensive stays and I'm at a point in that promo where a "hit" has an effective return of 5,000 extra miles.