Yes, the US is in a minority of countries that provides a government registration of copyright. In other countries other methods must be used to prove the time of creation as it relates to possible originality vs copying.
The main benefit generally to registration is that if you can prove you created your work prior to the claim of creation of the same work by an infringing party, he has a harder time proving that you stole his work. Registration helps fix the time of creation. Oddly enough two independent authors can theoretically both claim copyright to identical works so long as you can show that the second author had no knowledge of the first author's work, and thus that it was independently created rather than copied. In practice if one work has been published or distributed, there is usually a presumption that authors of subsequent works had the opportunity to copy. In that sense, there is a potential advantage in publishing to the internet, in that the appearance of the work online at a certain date 1) establishes that the work was in existence and claimed by the author on that date. and 2) lends credibility to a claim that an infringing author had the opportunity to copy. Copyright registration does not provide that latter presumption, since the material in the registration system is not generally known to other authors.
A clear disadvantage to the publishing of a song on the internet, at least under US law, is that once a recording of a song is publicly released, it is subject to compulsory licensing. That is, anyone can cover your song without your permission so long as he pays statutory royalties. So if you wanted to be the first to release your song on CD, someone else may be able to release his version before you can. In the unlikely event that you could get more than the statutory royalty for a recording release, you are unlikely to get that if the song is already subject to compulsory licensing.
You will need a US Copyright Office registration before you can contest infringement in US courts. Under US law a copyright registration that in a timely manner (registration within three months of the work’s publication date or before any copyright infringement actually begins), also subjects an infringing party to statutory damages and awards of attorneys fees in litigation, and in addition provides presumptive evidence that your copyright is valid.