Having played both the PS1 ToP and PS1 ToE...personally it's pretty equal. I've played both at their hardest settings and generally it's the same. ToP might be a bit harder because of the outdated combat system, making it sometimes clunky and unresponsive which made add a layer of difficulty. --- Grand Marshal/Gladiator Priest, Lightplol. PvE gladiator holy paladin, Lightp.
Tales of Phantasia by far and even on the latest version I still find it hard. Tales of Eternia was much easier, but some bosses are hard. --- "Don't let it end like this...Tell them I said something." Pancho Villa
That depends for Mania mode Dhaos I had its 2nd form via ToP X (build into the Narikiri dungeon X UMD) way harder to kill of then Neried.
But that's because during the attle in Phantasia Dhaos uses the same Revialitze spell as Mint and you better hope it never goes off else he restores 500,000 HP using it and that took me over 15 minutes to drain most of that at level 75 to 80.
So while Nereid may use 2x Shooting Star spells killing level 110 PC's all at once on Mania at least the boss has no healing spells to use at all.
Also its level 8 Rid Vs a level 1 Cless so Phantasia via Mania/Hardcore mode is stll harder to survive at the stasrt of the game because of him starting out at level 1.