I have a Camaro 1LT Convertible this weekend, and the trunk space is horrible for a few reasons.
1) the cargo shade (mounted in the trunk) must be pulled back in order to lower the top, and this decreases the trunk space. You might be able to squeeze some small flat objects under the cargo shade.
2) the trunk is basically a small well, and it's hard to maneuver large items into the trunk.
3) trunk is already tiny with the top down.
The Mustang trunk space is smaller, but more usable because a) no cargo shade, and b) the trunk lip extends to the bumper so easier to load objects.
Both have back seats that can reasonably store some luggage, though dunno about 2 suitcases and 2 carryons. My advice is to pack less if you can.