I am interested in the SSI system for learning languages. I plan to use this to learn Welsh, but for the coming months cannot commit set hours a week or money to doing so. I have decided to try the system out on a different language but am unsure if I should try Manx or Cornish, (I have looked and haven’t found these questions asked anywhere, sorry if I am wrong)
Would it be advantageous for later learning Welsh to have some knowledge of Cornish?
The Manx course seems to have a lot more lessons, is it more fleshed out than the Cornish?
You can do at least the first set of lessons / challenges of the Welsh course for free — I’m not sure how much of it, as I haven’t gone very far with it myself, but there will be others here who can tell you exactly. If it’s Welsh you want to focus on learning, that’s probably the best idea, so you can go right ahead and get a feel for the language itself and for how the SSi method works.
Cornish is closely related to Welsh, but they’re not mutually intelligible and the basic sentences are quite different in structure and vocabulary — I’ve done the whole SSiCornish course and started the (free) Welsh one, and knowing Cornish didn’t help very much with the Welsh at all, surprisingly. In some ways it could possibly make it more confusing if you’re new to both languages. Manx is very different from both of them, so it would be up to you whether or not you’re interested in trying it.
I could add that Cornish is the least widely spoken of all the surviving Celtic languages and needs all the help it can get, but that’s just a bit of promotion from me as a speaker of it! Again, I would think trying the free SSiWelsh lessons would be the most useful option for you.