Here is my research on both Pokemon to help :)
Haxorus has these moves it can learn after lvl 48 when it evolves from fraxure
Swords Dance
Guillotine
Outrage
Giga Impact
And it's stats stand at (these are just base stats)
HP: 76
Attack:147
Defense:90
Special Attack:60
Special Defense:70
Speed:97
With the base stat total 540
Haxorus has weaknesses of Dragon(X2) and Ice(X2)
Hydreigon has these moves it can learn after it evolves from Zwelious
Hyper Voice
Outrage
And it's base stats stand at
HP:92
Attack:105
Defense:90
Special Attack:125
Special Defense:90
Speed:98
With the base stat total:600
Hydreigon has the weaknesses of Ice(X2), Fighting(X2), Bug(X2), and Dragon(X2)
My thinking:
I think you should go with haxorous because it has a lower base stat total sure, but, haxorous has less weaknesses which would come in handy.
Hope I helped :)
