Well, this might not be the most popular opinion... but I liked the Sea Devils more than I liked the Silurians. Yes, the Silurians might have the deeper and more mature plot. And it is also played far more seriously. However, I just LOVE Jo and the Master, and the Sea Devils is a treasure trove for showcasing both Jo and the Master. Delgado is wonderful in this!! For me, he is even better than Pertwee here. Sea Devils is more light hearted, it is more comic-style storytelling, it has more humour and it is just more fun.
Whoahh - knife edge vote here, and I am not talking about today's commons debate. Bar the Silurian costumes and Pinky & Perky voices, the former is one of the most 'Adult' Drama of Classic Who - what else from Season 7 after all? But like others, the Sea Devils is just quintessential Pertwee Who and one of his most entertaining and dynamic tales, production and casting wise. Delgado really proves himself a co-lead here and not merely a supporting guest villain. So much charm that you can appreciate how he managed to take control of his captors will. And indeed we all feel relieved that he escapes to fight (play) another day. And it has Jabba the Hut...
Oof... It's a touch choice. I find The Sea Devils extremely watchable. It's probably Malcolm Hulke's strongest plotted six-parter, nothing is superfluous and the characters all feel engaged with the ongoing story. Plus, we get the novelty of an Earthbound story without UNIT. While the Master's villainy is front-and-centre for the first half, the titular creatures still make their presence well-known. The exploration of the sea fort feels ripped straight from a cosmic horror story and there's a great sense of adventure to the proceedings.
The Silurians, on the other hand, is a fascinating story to analyse. Far from the escapism of its successor, it's firmly rooted in complex themes and discussions that are still ongoing today. The titular people themselves are fascinating in how their society's formed, their expectations, even the costume is something I don't think Who's ever been able to quite replicate since. If you want to create a story with dark themes, there's one simple avenue you can take -- everyone makes mistakes. The plot of Hulke's seven-parter is about the alarming fallibility of human and reptile kind.
Between the two, it's The Sea Devils for me. I find The Silurians is just a bit too real at the moment.
"Courage isn't a matter of not being frightened you know; It's being afraid and doing what you have to do anyway."
I think everyone else has already explained the reasons why this is a difficult choice far more eloquently than I could. I think I'd have to go for The Silurians over The Sea Devils though. For me, The Sea Devils isn't as much of a fun romp as most of you say it is. It's something I've really got to be in the right mood for, otherwise I can find it to be a real drag. The Silurians, however, is one of my go-to stories if I want to watch some Doctor Who and have some spare time on my hands (I refuse to not watch Classic Series stories in a single sitting unless absolutely necessary, with the only exceptions being The Daleks' Master Plan and The Trail of a Time Lord, which I usually only watch pieces of anyway). I've always found it to be one of the most instantly enjoyable stories ever made, and it never feels like it goes for just shy of three hours. I just love it