I finally got both DS and DA to Level 151 and compared their DPS to each other both with buffs and without. I've also watched a couple of end game bossing videos which you can find in the External Resources section of the 2 links below:

Class overview and skill set of Demon Slayer here.

Class overview and skill set of Demon Avenger here.

In summary:

Demon Slayer

• Great single target and multi-target bossing at late game (Level 170 ) and very good mobbing capabilities. Mediocre class until you unlock Level 150 hyper.
• Decent survivability
• Overall very well rounded and the best Warrior class in KMSM

Demon Avenger

• Okay bossing capabilities and excellent mobbing at late game (Level 170 hyperskill)
• Survivability is unrivalled- he won't even need HP pots and he has ridiculously high base HP
• He has flashier / more aesthetically pleasing skills

Comparison between DS and DA in bossing:

• DA is simpler and requires less micro-managing of skills
• DS excels when there are no buffs
• DS starts losing his advantage once both classes are at 100% crit rate but still comes out ahead slightly
• In sustained battles or bosses with constant stuns/wipes/reflect such as Pink Bean DS suffers because he can't fully utilise the uptime of his infinite DF buff and thus he frequently runs out of DF spamming Demon Impact. DS also suffers more heavily when buff wiped due to having his Level 150 buff Blue Blood ( 55% additional damage) wiped and it has a long cooldown. By contrast, DA’s shields are able to continue attacking the boss regardless of stuns/reflects and his short Lv 150 hyperskill buff comes back quickly after being wiped
• DS will have no more DF issues at Level 170 and can spam his main bossing skill with 100% uptime instead of 75% of the time when I tested it at Level 151

Hope this helps some of you guys if you're thinking of prepping for either of these two classes!