We can compare characters from different timeskips since both have a reasonable amount of feats and scaling to back them up. There's no speculation here, just cold hard feats.
NOTE: This is just an analysis of feats. Hype-wise, Ajeel has this, but feats change this.
Analyzing Ajeel: He has 4 named attacks which are one small town buster (Ant Lion Pit), one town buster (Sands of Death), an attack that can potentially decapitate town level targets (Ramel Fa'as) and Ramel Sayf, which is almost featless. None of them can affect Mard due to their nature (Etherious Mard can fly and has more than enough reaction speed to dodge them) and their power.
Then Ajeel also lacks dehydration feats. His best dehydrating feat, and only one as far as I'm concerned, was dehydrating Gray's ice, which probably means that he can dehydrate Mard's thorns. However, his inability to affect Erza, a character with town+ durability, means that he won't affect Mard either. His only real chance would be by casting Sand World, which would grant him speed advantage, but that's it.
Ajeel's physical feats are restricted to being scaled to Erza - small town level - and being unable to tank MCB busting attacks, which would put him around MCB overall physically.
Analyzing Mard: His thorns and durability can be scaled to CSK, which would put his Etherious Form around City+ level in both durability and DC. However, his thorns are nullified by Ajeel's dehydration (except for Yggdrasil) and his intangibility, which also completely nullifies Mard's physical strength. On the other hand, Mard has HS+ speed, which puts him above Ajeel without Sand World and slightly below SW Ajeel. Furthermore, Mard also has Plutogrim, which is potentially City-level and might ignore intangibility, and Memento Mori, which is presumably City level via hax, but is featless, which makes it not applicable in this match-up.
Veredict: Mard wins high diff with Plutogrim, but without it, Ajeel will ignore all of Mard's attacks via durability. Seeing that Ajeel doesn't have anything that can damage Mard either, this will end up going by a stamina/MP duel, which I presume Mard wins. Wither Mard wins extreme diff or it's a draw.