Marshadow is better than Gengar, and by a long shot. It is arguably better than Mega Gengar as well. But I'll focus on standard form Gengar as per your request. Some things that make Marshadow favourable include:
Fighting and Ghost is a great offensive typing. It gets perfect neutral coverage (i.e. nothing resists it). Gengar doesn't have this, and it hits less for super effective damage as well.
Marshdow is slightly weaker than Gengar in its preferred attacking stat, but it is much faster. It hits a great Speed tier at 125 that lets it narrowly outspeed Pokemon like base form Greninja.
Marshadow has a better ability than Gengar. Technician boosts Shadow Sneak and Hidden Power Ice, which each restrict safe counters to Marshadow a lot.
Marshadow has very serviceable bulk of 90 / 80 / 90. This makes it harder to consistently OHKO, let alone revenge kill on top of its Speed. Gengar's 60 / 60 / 75 is poor.
To put Marshadow's Speed and bulk into perspective, the only Pokemon relevant in OU that can safely and consistently outspeed and OHKO Marshadow is Scarf Tapu Lele.
Marshadow lacks defensive counterplay. Nothing resists its STAB as above, and its Marshadium-Z + Bulk Up can OHKO physical walls with minimal setup.
Spectral Thief is a great asset. It is a strong attack, and its side-effect means Marshadow's mere presence denies any set-up sweeper that doesn't boost Speed.
Marshadow has a lot of very effective sets that are countered differently (Choice Scarf, Swagger, Marshadium Z, etc.) This is less the case for Gengar.
Marshadow has as many good movepool options as Gengar, including Ice Punch, Stone Edge, Poison Jab, Will-O-Wisp and others. It can literally pick and choose what it beats.
Note, Gengar is still a very good Pokemon and a potent threat in both the metagames you mentioned. But Marshadow is on a whole other level: it was banned unanimously by Smogon's OU council and is one of the top three Pokemon in Ubers by usage. You will not find a better Pokemon that is still allowed in OU.
