D2 is better than D1 in pretty much every single regard. The only area where the two can exchange blows is music. D1 has better map music, D2 has much better combat music (and also more than a single track that gets old pretty fast in D1).

The fact the you can't even level units past their last evolution in D1 makes it pretty boring to play too on longer maps, where battles just become a chore you gain nothing from. The campaigns are also like 4 missions per faction. It can serve as a good intro, but after you've played it once, there is pretty much no reason to play D1 over D2.

And as for balance in Heroes 3, it has never been and never will be balanced. The amount of restrictions, changes and special rules that had to be put in place to make it even remotely competitive for multiplayer games is evidence enough. Necropolis is busted, Conflux has been the biggest balancing joke in gaming history since its release, caused by the hurried dev cycle because the original faction they've been working on had to be scrapped and they just glued together Conflux at the last minute. Spell balance is a complete mess, Skills are either mandatory or completely useless, most hero specializations are useless and so on...

Yes Heroes never be good balanced. And there is more tactics in Disciples no moves battles (maybe it can be improved by allow units swap positions at cost of turn).

Disciples 1 have better EXP system. Or differ... It lack of upgrades for neutrals. Should be second form or one level up. Units in campaign limited in upgrades should still get levels up to 3-4, but they not. Units in first mission literally limited to Level 2. That's bruh.
But in Skirmish that system completely change gameplay. Make it BETTER. Large maps and Endgame are much interested when you not strict to one Uber party against another one Uber. It can be named better for sure at 100% 2x than D2/ROTE if not campaign and neutral issues. And no lack of auto healing (for neutrals too).
Disciples 1 was unfinished in such details. Wish Disciples 2 Alpha be available to public to see how far it changed in that details.

Thank you for your detailed post! You confirm much of what I've read on the matter (also concerning balance in HoMM2&3) and again, my focus is on story-driven campaigns, and it's clear D2 is better than D1 (thanks again to QWEEDDYZ for the numerous inputs too) so even before starting the game I know I've made the right choice with Disciples II Gold :)