Great, bom caminho!
A very quick comparison between the Caminho do Mar and the standard CP route Lisbon-Tomar is: the Caminho do Mar is longer, has better 'sites' in my opinion (esp. early coast, Sintra, Óbidos and Alcobaça), more asphalt, and less pilgrim infrastructure (e.g. no albergues).
For a longer discussion, we posed this question -- is the Caminho do Mar to Fátima and then the link Nascente route to Tomar a viable alternative to the standard CP to Tomar? -- at the end of the second episode of our
podcast series on the Caminho do Mar (ep 4.2).