I'd personally use them for different scenarios and different points during a game of survival.
It's also important to note that they have different uses depending on the map you're playing.
As you reference Seatown, I'll talk about that first.
I tend to use the predator only on the earlier rounds (<30) due to the limitation you are aware of - the inability to move. Points are abundant on this map so it's easy to use a predator to wipe out the early juggernauts if you can get them to bunch up together.
For juggernaut rounds >30 I prefer to use airstrikes solely because you can 'fire and forget'. Moving around the map is very important on Seatown in later waves, so this is an immense benefit.
The other good thing about the airstrike is that you can team it up with flashbangs to keep a group of enemies around the spot that the strike is called in. This obviously works best with a coordinated 2 player approach - one with flashes, one with airstrike.
But, on Underground the predator is extremely useful in all rounds (especially from 30+) to hit a group of enemies at their spawn point on the raised platform by where the two trains are parked just before the entrance to the building. (Use a predator to survey the battle field and you'll see where I mean, I'll try to post a link to a map later)
Hitting the enemies here immediately after spawn allows you to get quad kill/kill streak/rampage quite easily and the predator effectively pays for itself. Again, I find points quite abundant on this map, so it's not as important to be conservative with them.
Remember, if you are hitting round 39 now, to improve you want to get to round 45 or 50 then build on that. There is no point dying with $50k that you could have spent on airstrikes, predators, riot squads, body armor etc.
It's important to keep a reasonable amount back for self-revies, claymores, c4 and flashbangs - but otherwise you can spend more than you think.
For reference, I play on PS3 and currently sit around #500 for Underground (round 62).
I think my Seatown best is round 51, but working on bettering that currently.