A live well pump and a wash down pump are created for different purposes. live well pumps are made to pump hi volume at low pressure
.........wash down pump is made to build pressure so it doesn't produce as much volume
also, as you said, a wash down pump will use more power, especially if you run it constant. And if you are running it constantly a live well pump is cheaper and easier to replace.
Last season I put in a wash down pump and live well, I too was toying around with using the wash-down pump for both (I have a much smaller live well) I then came to the conclusion that a live-well pump cost like $10 more than the Y valve, It just didn't make sense.
Lastly, And I think most important, That's a big freaking live well. If you don't have a decent pump it is going to take for ever for that thing to fill up. you'll end up playing the bucket game again.
You know what you have to do
Oh Yeah!!!!!!! Don't hold out on were you got the barrel man