Are there any authoritative sources regarding which language should be tackled first?
Based on this article on elvish.org, the very simplest answer is that you can't. Neither language is sufficiently complete as to allow someone to actually learn to speak them conversationally, something that Tolkien himself addressed in his letter #380;
"It should be obvious that if it is possible to compose fragments of verse in Quenya and Sindarin, those languages (and their relations one to another) must have reached a fairly high degree of organization — though of course, far from completeness, either in vocabulary, or in idiom... It must be emphasized that this process of invention was/is a private enterprise undertaken to give pleasure to myself by giving expression to my personal linguistic 'aesthetic' or taste and its fluctuations"
That being said, there are various resources that will allow you to become as knowledgeable in the "elvish languages" as anyone else (including dictionaries and lexicons of Quenya and various online courses in Sindarin) but given their dissimilarity with each other and with spoken English, the fact is that there's no major benefit in learning one over the other, nor will learning one first give you a dramatic advantage in learning the other.
I'll post my own opinion on the matter..
I've been learning on my account both Sindarin and Quenya for a couple of years; there is enough material for both of them, so don't worry for it (Use Fauskanger course for Quenya and Pedin Edhellen for Sindarin, both are excellent texts).
Now, about them:
Both languages are gonna be a bit complex because of their grammatical setup; Quenya is based on Finnish (which comes from the uralic-root) and Sindarin is based on Celtic (Celtic-root); mostly of European languages (English, Spanish, French, German, etc.) are based on the indo-european-root, so they are 99.9% gramatically the same. Said that, Quenya is a bit difficult, Sindarin is a hell.
The difficult part in Quenya are the declinations (if you're a spanish/german/french speaker it won't be so difficult, because we have also a lot of declinations in our languages) and in Sindarin are the mutations (there are very very very few in english).
Now, in my opinion.. Quenya is a "Full-Language", meaning, you can express almost anything you want (gramatically speaking), while Sindarin is very weak in that aspect; instead of '100%-specific' phrases, you end up expressing 'ideas' which have to be understood and interpreted by the context (which, again, in my opinion, make it loose a lot of points..).
Making an overall evaluation.. Quenya can fully compare to a nowadays language; you can comunicate everything you could in your own language, while Sindarin is like a 'stone-age' language; it can be used to comunicate basic things easily, but if you try to make more complex sentences, you will see a lot of precission loss and a mess due to the mutations.
I believe the reason why people learns sindarin is mainly because it's the language spoken in LOTR dialogues, because, choosing blindly, i would choose Quenya 100 times.
If you have any other questions, i will be happy to help! :)