What is the difference between a slab coil and an "A" coil of the same size/capacity and what the benefits of either?

We are in the process of replacing our upstairs AC with a new condenser and coil. Most installers have quoted A coils (which we have now), the last quoted Slab coils saying they were better, but not really giving me a reason.

There is plenty of space in the attic where the current coil

Help is appreciated. I am trying to pick a system this weekend.