Neither of these is remotely comprehensible without knowing much about the back-stories concerned - with Happy Endings, it's the previous 40 New Adventures featuring Benny and her travels in the Seventh Doctor's Tardis with Ace, Chris Cwej and Roz Forrester; with Many Happy Returns, it's an even larger number of audios and books featuring her involvement with the historical collection of the urbanely sinister Time Lord, Irving Braxiatel. It's a bit invidious to compare the two, as they are works for different purposes in different media written in different centuries. But I will do so anyway.
Happy Endings was the 50th New Adventure overall, and the 41st with Benny. It's a rave-up celebration of the series so far: Bernice is marrying Jason in the village of Cheldon Bonniface, and many characters from previous Who stories, particularly the New Adventures, turn up either as guests or as potential spoilers. Paul Cornell is always great when expressing his love for Who (or indeed other things that he loves) and this is awfully good fun, particularly for the further characterisation of Benny as uncertain bride.
It was interesting to read it so soon after Last of the Gaderene, which takes itself much more seriously, and is about the same length, but seems to have only about half the plot. I cheered at various points, though I confess I also scratched my head at others as characters who I barely remembered from their single appearance in a novel I read three years ago emerged blinking back into the narrative light. But the cheers were more numerous than the head-scratchings, and really, what more can you want?
Many Happy Returns is the work of many hands, and gets off to a very tricky start with a misfiring postmodern sketch - but after that it livens up considerably, and although Big Finish have done the theme of telling a story through a key character exploring a dodgy museum dedicated to his or her life life a couple of times now, this really takes off through the performance of Lisa Bowerman and the rest of the company; perhaps the change of pace to what is essentially a sequence of five-to-ten-minute sketches was stimulating to the creative juices all round.
Anyway, I could not really recommend either of these to anyone who was not already a fan of that particular branch of Bernice Summerfield continuity; but for those who are, they are indispensable.