As far as which axe to get if you do want to go that route the MDB ones are a no brainer. MDB and PDT have increasing gains the more you stack. for example if you have 80 MDB and add 10%, you decrease damage taken by 50% if you have 80 MDB and add 15% MEVA, you decrease magic damage taken by 15%
Although this is strictly true, it's important to note that MDB +X is not the same as (Magic) Damage Taken -X%. As far as I am aware Ducal Guard does not give MDB+50, it gives -50% DT (If I am wrong on this then please correct me).
Each point of MDB reduces the magical damage taken in the base calculation formular by 1%. Damage Taken -X% reduces all damage you take from magic sources by a flat X%, which as a seperate calculation does not directly stack with MDB. The two are applied to magic damage as seperate factors.
Adding 10 MDB to 80 MDB does indeed reduce remaining damage taken by 50% for a grand total of 90%, but damage reduction atmas are not the same as MDB atmas. So a more practical example is that you are adding MDB+10 to -80% MDT. What actually should therefore happen is this:
1) MDB+10 alters the damage calculation formular by 0.9 (1-(0.01*MDB)) - Damage reduced to 90% base.
2) -80% MDT reduces remaining
damage to 20%, so that's 90%*0.2 = 18% of base.
So adding a MDB+10 axe to a high MDT build will have only a minimal impact.
This is not to say that MDB axes are useless, but they do not factor into a damage reduction build as well as the -PDT axes do into a PDT build, and more specifically 80% MDT plus MDB +20 wont give you 100% MDT, it'll give you 84% MDT.
In a pure MDB build they are great addition though; if you have no DT to factor in then every 1 point of MDB is a flat 1% of total magic damage reduced. As such, to maximise the effectiveness of your axes you want to pop on some MDB Atmas instead of damage reduction atmas (unless you can reach a higher total MDT than MBD, in which case you should do that and maybe consider switching the axes for something more useful). Atma of the Future Fabulous is the only high MDB Atma I'm aware of though, so overall you're almost always going to be better stacking MDT on top of Ducal Guard, as you'll get 50% PDT from that too.