Unsure which Gengar to power up or to TM (if at all)

The dust investment between level 20 and level 23 is negligible. You get synergy and approx. 10% higher DPS with Lick/Shadow Ball over Sucker Punch/Sludge Bomb.

Gengar 2. Theres no doubt here.
Your 100% Gengar is sadly just a trophy. Unless you didnt catch/evolve enough Lick Gengar to have a full team of them.

Your gengar 2 probably already does more damage than gengar 1 even with the CP discrepancy.

The 98% with Lick no question. Without a way to get Shadow Claw or Lick on your 100%, it's sadly obsolete.

Gengar #1 will serve you better with Sludge bomb against fairies and other stuff weak to poison. It also is effective against fighting attacks, with Gengar's triple resistance, but SB does more DPS.

Gengar #2 is the one to power up. Lick/SB or SC/SB are the only efficient way to use Gengar against stuff weak to ghosts, and given Gengar's relative frailty, you need to make those few attacks count.

The IV difference is negligible over the long haul. The moves are paramount. Remember, it's always been Species - Level - Moveset - IVs in terms of importance (most to least). If you doubt it, lets fight my level 15 Raikou against your level 40 Magicarp, or my level 40 Mewtwo with Fire Blast against your level 20 Mewtwo with Shadow Ball, or my level X 0/0/0 Gengar with Shadow Ball against your level X 15/15/15 Gengar with Sludge Bomb.

If you had to choose one of them, I would choose Gengar 2.

Over time, Lick will be the better choice, offering better dps and to compensate players who didn't get Shadow Claw. You should also chance the tm to change Psychic into Shadow Ball. I managed to get 2 of my Gengars into Lick/Shadow Ball so far. (I need more Charged TMs!)

Hex *compared* to the other fast moves are inferior.