If you're staying onsite, in one of the DL hotels, it doesn't really matter. If you're not, get them from GetAwayToday, because then you'll get 2 Magic Mornings instead of just one. (if you stay onsite you get to use every Magic Morning; it's an unadvertised perk of staying onsite.)
Also, if you are staying onsite, & have a room-only reservation, add the hoppers to your reservation. You save some $ that way instead of buying them at the gate. Not sure if it is the same price, more, or less than getaway.com.
If you want more than 5 days, get them from GetAwayToday. Their 5 day hopper is the same price as buying from DL. They offer a 6 day hopper for just $5 more. They also have a 7 and 8 day hopper that is just $10 more than the 5 day hopper. As pointed out in the previous posts, they include 2 MM rather than the 1 that comes on DL's 5 day hopper.
If you buy from GetAwayToday and are staying at one of Disney's hotels, you can pickup your tickets when you check in at the front desk. Not all CM's seem to know they can do that. When we checked in, I showed him the email confirmation from GetAwayToday and asked if I could get the tickets. He wasn't sure but went in the back to check and came out with my 8 day hoppers. It was very convenient and having the extra 3 days for just $10 more was a bargain!