Tentacruel is weak to Electric, Psychic, and Ground but resists Water, Ice, Fairy, Poison, Bug, Fire, Steel, Fighting. 3 weaknesses, 8 resistances.
Venusaur is weak to Flying, Fire, Psychic, and Ice but resists Water, Grass, Fairy, Fighting, and Electric. 4 weaknesses, 5 resistances.
It's all type synergy, really, so Venusaur might fit some teams better, though Tentacruel has a better set of resistances IMO. Though as a Regenvest, Venusaur is vulnerable to Fire/Fairy coverage, which is nastier than Ground/Fairy due to moves like V-Create and Blue Flare, and being vulnerable to two -ates isn't doing it any favors. On the other hand, Megasaur does have a stronger offensive presence, so I have little doubt it has some options viable to it that Tentacruel can't do.
As for species clause, I think I've said before I'd be totally okay with it. And AFAIK, species clause works off of dex number, hence why you can only use one Rotom form in standard. So regardless, that'd likely force players to choose Xtwo or Ytwo for their teams since they share the same dex number. Plus teams spamming six of the same mon and playing "lol guess the set" gets kind of old and annoying. And beating them because you happened to have the one SE sweeper against them isn't fun either.
Also, though I'm not sure it'd happen even if we got species clause, it could let us bring back plateless, Multitypeless Arceus forms without half of everyone's team being comprised of different Arceus forms.