Of the two, I generally find that Quenya is easier for beginners. It is a more “complete” language in the sense that we know more about it and have a larger vocabulary for it. It’s grammatical rules are more straightforward.
Sindarin has a complex set of sound mutations that can be difficult for English speakers to learn, unless they are familiar with real-world languages that have similar rules, like Welsh. However, within Tolkien’s world, Sindarin was used more commonly than Quenya, being the ordinary daily speech of most Elves of Middle Earth. Sindarin appears more often in Peter Jackson’s movies, for example.
So I generally recommend Quenya first if you just want to learn any Elvish language, but if you want to role play or write fan fiction in Middle Eath, Sindarin is more practical, since Quenya is an ancient scholarly language rarely spoken, like Latin in the real world.