I'm aware of one thing ludus can mean that lusus can't: "school", as in ludi magister = schoolmaster.
Ah. Well, neither the OLD nor L&S mentions that meaning, and if that disputed example is the only one we have, we can can consider that lusus perhaps could, rarely, mean "school" but that there's no certainty about it.I thought so, too, (and I suppose if you want to say "school", ludus is definitely the better choice) but my German dictionary gave me this (part of an) entry s.v. lusus (with a discussion at the end that somebody considers lusum a supine in said passage):
B) objektiv = die Schule, Lektion, lusum dimittere,die Lektion schließen, Varro sat. Men. 279 (vgl. Suet. gr. 6 u. 18 dimittere scholam; dagegen nimmt Döderl. Synon. 2, 28 lusum als Supinum von ludo u. ergänzt zu dimittat das Objekt ipsos).