A518 vs 727

My transmission is a 727 that is pretty leaky and has some issues. That being said, I wanted to get it rebuilt or a better transmission rebuilt and put it back behind my 318 in my '74 D100. Here's what's going on. If I get a 727 or an A518, it'll be the same price to get them rebuilt with a nice shift kit. $800.

So which should I go for? I wanna get pretty decent gas mileage but still be able to romp it and have some fun. I have a junkyard full of good cores to chock up.

What's the disadvantages of each?

Which one is the best overall?