For my own interest, I have been comparing the Joanna Yeates case with the Becky Watts case for some time. Both crimes occurred in Bristol and were tried at Bristol Crown Court. Both defendants pleaded guilty to manslaughter, and were subsequently found guilty of murder. That is where the similarities end.
Becky's father wrote a book about the case (his name is Darren Galsworthy, but I cannot remember what the book is called). It was clear, from what he had to say, firstly, that Nathan Matthews did not like Becky (probably, he was jealous of her: his mother was Becky's stepmother, Becky had suffered from anorexia, and had received more attention from her stepmum than Nathan would have liked, IMO).
Secondly, Nathan had a history of being sexually interested in underage girls. Even his co-defendant, Shauna, was underage at the time they first met. His stepdad was very unhappy about this, and banned these girls from the house until they were sixteen.
Yes, the police interviews with Nathan and Shauna are available on line. The CCTV of them buying rather "incriminating" goods is online too, and has a timestamp as well as a date stamp.
All in all, Nathan Matthews had a motive to kill Becky, and what we see him buying at B&Q (I think!) provides rather good evidence that he had committed the crime.
Vincent Tabak had no motive to kill Joanna Yeates, (at least, as far as we know), and there is nothing suspicious about a man buying beer and crisps in ASDA.
And, of course, we have never been able to watch his police interviews.