It's my understanding that the "Ducks" were faster both in the water and on land... please correct me if I'm wrong. If so, sometimes speed is more important than armor.
Frankly the LVT is easily killed with 50 cals... so it's defence really isn't all that great. True it would be better than the duck, but either way you're gonna get nailed.
I think the Duck would be used quite a bit...but that's just my guess.
Keep in mind though... the above opinion was written under the influence of beer...lots and lots of beer.