Ok i know this has 2 answer, even a best answer but im going to use a Competative view and just go straight out and say it Terrakion is the way to go.
Credit to hex for the image.
Okay so i dont need to do much pros and cons for the two as ive played the Ou meta game for half a year now and i know almost there is to know about it.
And i will ask you now, Haxorous may have good atk but can it abuse it.
Look at its speed 97, pretty good eh wrong it is far outclassed by half the tier and more importantly Salamance.
97 Speed is just what this guy didnt want 101 maybe it would be used more but even with a scarf it is easly countered due to its terrible def which isnt great paired up with low flexibility.
Its movepool you could say is similar to Terrakions but Terrakion can pull it off so much better.
Haxorus's attack is almost equivalent to Zekrom's
With no disrespect to the other answers (this question is all opinionated) 147atk is almost equivilent to Zekroms but does that make it good.
No just powerfull, look at Shuckle the average def is around 70 his is 230 what tier is shuckle in? Nu was* it.
Terrakion has well rounded stats with good speed and great attack, yes it may not be as good as haxorous's but lets put it this way.
If Haxorous had 10 less base atk it would be UU possibly Ru maybe it is almost never used in OU.
Whereas Terrakion is the best revenge killer in the game no one can take a stab close combat very easy well appart from Jellicent.
108 Speed is a god send making it very hard to counter without the use of priority mach punch or Starmie.
It has the ability to wreck in all weathers and a choice band set is one of the most deadly sets in Ou, Choice scarf is more common and makes it extremely hard to tackle.
Terrakion has also a much better movepool and it can abuse almost counter any situation.
For this reason i and many others (mainly experienced battelers) will agree Terrakion is the way to go.