Agreed. Spacey had an extra layer of sinister to him and really wanted some cold revenge on Superman. Hackman never really seemed all that irritated with Supes other than "You foiled my plan! Rats!" Killing Supes always seemed like a secondary measure, if possible (and if not, no biggies).
Hackman did have a couple moments that showed how cold and heartless he could be ("no, by causing the deaths of innocent people"), but yeah, mostly it seemed like just a game to him.
Although in some strange way, I have to admit his portrayal of Lex also feels kind of right to me. At that stage in their relationship, it makes sense that someone with his ego and intelligence probably WOULD regard Superman in a dismissive way like that, and as nothing more than a nuisance.
Whatever the case, I will always find Hackman's performance in those movies to be absolutely brilliant and fun as hell to watch.