I read both but Malory Towers is the better series imo. Perhaps it's the fact there's a book for each year, maybe it's because I read them first, maybe it's because I secretly wished I went there... Darrell just seemed more rounded as a character, what with fighting Ellen in the middle of the night and slapping Gwendoline and allowing West Tower girls to a North Tower picnic. All the twins seemed to do was look down their noses and then become secondary characters because actually, they weren't that interesting.
I always thought I'd I went on Mastermind, Malory Towers would be my specialist subject, I've read them so often!
Anne Digby (I think) wrote the rest of Felicity and June's time at the school, another 4 or so books. I've read them, they're watered down like the modern edits of MT, but very much in keeping with the originals.
I have my 1980s set, still in fairly good condition, just in case I ever have a daughter (and I re-read them myself every now and then!
).
BTW, Jenny Colgan had written two of a series of modern boarding school books but more from the teachers' perspective. They're okay, but some of the things that happen are too much like things that happened in the MT series (like when Jo and Deirdre ran away and hid in the shack). The teacher side of them is very interesting though.