What is better?
Doing duolingo lessons on PC or on android phone? Where will I have greater progress in learning languages?
Probably PC because the exercises are more varied and a little bit more difficult so you'll learn more.
There isn't necessarily a correct or incorrect solution to you're question; it mostly depends on which of the two options a user might work better with. Some people might progress more easily using one platform over the other. It's very debatable; if I'm thinking correctly, the website version offers a Tips and notes section (something Android and iOS don't offer), but Android has Japanese available as a course while the website doesn't yet.
Unless you want to learn Japanese right now, I'll say the PC.
I haven't tried the app, but is it true that only the PC version offers tips and notes?
It depends how you use them. At the levels of progress in trees where the vast majority of users are I've found a lot more translation into the target language on the app. If (and it's obviously a huge if) you actually do the translations for yourself before relying on the word boxes at the bottom of the screen, I could see an exception to the general observation that you'll make better progress on the web.
Well, the PC is more difficult and you write much more, so it really helps to test your knowledge. However, the app does have more native to target translations which are obviously useful. Plus, I have found that when I really struggled with a lesson not only do I get less frustrated when using the app because it is a bit easier, but it gave me different sentences than the PC so I wasn't just memorizing the sentence, but actually learning the words. You also get bonus xp on the app if you make few or no errors (up to 5). So I like to use both. I do the app when I am first learning a skill and then when I feel fairly confident that I understand it, I switch to the timed practice on the PC to cement the lesson in my brain.