The Corinthian was published in 1940. Sprig Muslin was published 16 years later, in 1956. As was the case with many of Heyer's early romances, the age gap between hero and heroine has not worn well. In fact, the similarities of the plots in the two novels makes me wonder if Heyer herself didn't rethink the happy ending from The Corinthian.
The stories themselves share significant similarities - in each of them, the ingenue (Penelope (The Corinthian/TC) and Amanda (Sprig Muslin/SM)) are fleeing from home (Penelope to escape a marriage she does not want/Amanda to manipulate her grandfather into consenting to a marriage she does very much want). Penelope is 17. Amanda is 16.
The heroes are also quite similar: Gareth (SM) is in his late twenties/early thirties; Beau Wyndham is 29 (TC). Both are icons of Regency style.
The Corinthian contains a plot device related to a highway robbery, which results in the theft of a set of notable jewels, which propels the action. In Sprig Muslin, as well, there is a fake highway robbery, and, of course, Gareth ends up getting shot (although it is a mere flesh wound).
The main difference between the books is in the details of the primary romance. Wyndham & Penelope are not at all convincing as a matched couple. He regards her, more or less, as the darling toddler gamboling on the carpet and making him laugh at her adorableness. There's no sense of the two of them meeting on equal footing. She amuses him, but the notion that he finds her sexually appealing is almost too much to bear.
He, on the other hand, is much too old for her. It is squicky - he plays the role of uncle & tutor. She is the school girl falling in love with the extremely attractive English teacher. She is blinded by infatuation. He seems merely creepy.
In Sprig Muslin, on the other hand, Heyer abandons Gareth as the hero for Amanda, pairing Amanda with an age appropriate young man and giving Gareth a heroine who is more than his equal in Hester.
So, while the writing in The Corinthian is, as always, nearly perfect, the romance itself is vaguely painful and regrettable. Penelope will never be able to grow up fast enough to satisfy what Wyndham needs in a partner. This romance is doomed to failure, and the marriage will be an epic disaster.