Pretending for a second Mercer and Holtz are at the same point in their respective progression arc, I feel like Holtz would be the only one "hurt" by being rushed. His game seems like a top 6 or nothing game, if you shove him down on the 3rd or 4th because he's not ready for top line minutes, he will have to learn to play a different type of game while ignoring the skills that he should be working on. Mercer is more like a Coleman, put him anywhere and he will get shit done. He seems like the type of guy who can work his way up the line up and change his game along the way. He might not have the ceiling Holtz does, but his style plays better when tossed around the line up.
I would say that Mercer has the ceiling Holtz has in every aspect except goal-scoring. Both players have excellent passing vision, better than they are given credit for. Mercer is the better prospect in pretty much any other respect -- two-way play, physicality, intangibles.
Both Holtz and Mercer are high, high-end prospects. Holtz has rare goal-scoring upside and the potential to also become a very solid all-around player in addition to this. Mercer has the ability to be an absolute beast in your top-6, whether it be as an all-situations 2-way center or a high-scoring, 200-foot-difference-making RW.
You are correct that Mercer's versatility makes it less worrisome to play him in an NHL bottom 6 role to start his professional career. In the WJC, Team Canada utilized Mercer as a 4th line RW in the 2020 tourney and then as the 4th line C in the 2021 tourney and Mercer shone both times as perhaps the best player in Canada's bottom 6. Holtz is not a bad all-around player, but he clearly needs more work in his 200-foot game and likely needs a top 6 role to thrive and develop.
Personally, I would like to see both bright young prospects start the year in the AHL, as I prefer to a more patient outlook with a top prospect's development. However, if Mercer proves he is the Devils best candidate for 3C, it is certainly the biggest hole right now in the line-up. Though the Devils seem content to play Zacha in that role, Zacha has proven to be far better on the wing than at C -- and if you take Zacha off the wing with Hischier/Bratt there is cataclysmically huge talent drop-off to the next candidates of Johnsson or Vesey or whomever... unless of course you were to play Holtz in the NHL which would allow Tatar to play on that Hischier/Bratt line.
From what I've seen so far, I'd say the optimum Devils line combinations might preclude my desire to be patient with prospects and look more like so:
L to R
Sharangovich-Hughes-Tatar
Zacha-Hischier-Bratt
Vesey-Mercer-Kuokkanen
Wood-McLeod-Studenic
spares: Jankowski and Johnsson
