klaymen: Heroes2 expansion gives you several new adventure objects, artifacts (and one set) and a campaign.
Heroes 3 expansions besides campaigns contain one imba castle (no, Conflux is hardly balanced), some new neutral monsters (even some from Heroes 2 like mummy or nomad) and possibility to combine certain artifacts to sets.
I have probably forgotten to mention something, this comparation is only what I remember off top of my head so take it with a grain of salt.
But to ultimately decide, I'd pick that one which you like more. You can't pick the wrong one, both are great.
Heck, gimme yer mail and pick one of them, I'll gift you the other.
I guess I will prefer Homam 3 (My first Homam game :) ), my email is Kluz.L@seznam.cz and you dont have to give it to me :) sure I would be happy as hell, but I guess Homam 3 will keep me busy for long time :)
Homam 3 looks really neat with all expansions and stuff thanks for tip :) Homam 2 will be probably my next buy, when I will have some money.