Thinking about the Old School DVDs and Sesame Street Classics on iTunes, it seems like Sesame Street Classics has a more random selection of episodes. Of course it helps that the Old School volumes all stick to the format of season premiers (and quite a few of them do seem like notable episodes, even without their status of starting the season).
With Sesame Street Classics, the first and tenth season premiers are the only ones included (I'd like to say maybe they were preparing in case more Old Schools were released, but then again, those two were also on the Old School sets), I think the only other really noteworthy episode included is Telly's debut episode. Volume 1 included episodes from season 20, they could have included the episode where Gabbi is born, and volume 2 includes episodes from season 15, they could have included episode 1839. There are some episodes from season 25 that seem noteworthy (like Susan and Gordon staying at the Furry Arms Hotel), though maybe just to me. For season 1 episodes, they could have included the episode where Big Bird wants to go to day care, which is what led to Caroll Spinney deciding Big Bird is a child.
Of course Sesame Street Classics does include a lot of great classics that have otherwise never been released on video (I have difficulty considering them video releases, I feel I'd be more accepting if it was a physical release or a specially-assembled compilation), they've given us a lot of episodes that didn't have information on the wiki (all the season one episodes had some info on the wiki, very few of the others didn't have so much), and we also got a number of sought-after inserts (whether we just needed to see them in English or at all) and inserts we'd never heard of (and there are some that, while I wasn't really familiar with their existence, I took notice of because of the releases), more rare stuff than the Old School sets gave us (of course I'm not talking about the bonus "classic cuts", though I think Sesame Street Classics even has those beat in terms of former rarities). And both Old School and Classics do contain a lot of well-known classics that many fans are familiar with.
If the Old School releases were a random assortment of old episodes as opposed to just season premiers, the episode selection might feel just as random. Or maybe they'd primarily include episodes fans would expect (the first episode, Mr. Hooper's death, Maria and Luis get married, Snuffy is seen, maybe Miles getting adopted, maybe Gabbi being born, maybe some episodes that feature a celebrity in the main plot, maybe selections from some of the multi-episode trips). And if there were to be a new boxed set of classic Sesame Street episodes (one that doesn't follow the Old School format), I wonder if they would just be physical releases of the episodes in Sesame Street Classics (and maybe some of the '90s episodes currently airing on HBO).