What are the advantages (or limitations) of one over the other for transferring files over the Internet?
(I am aware of secure forms of both protocols. I'd like to hear comparisons through personal experiences in terms of performance, reliability, file size limitations etc.)
Heres a performance comparison of the two. HTTP is more responsive for request-response of small files, but FTP may be better for large files if tuned properly. FTP used to be generally considered faster. FTP requires a control channel and state be maintained besides the TCP state but HTTP does not. There are 6 packet transfers before data starts transferring in FTP but only 4 in HTTP.
I think a properly tuned TCP layer would have more effect on speed than the difference between application layer protocols.
Heres another good comparison of individual characteristics of each protocol.
One consideration is that FTP can use non-standard ports, which can make getting though firewalls difficult (especially if you're using SSL). HTTP is typically on a known port, so this is rarely a problem.
If you do decide to use FTP, make sure you read about Active and Passive FTP.
In terms of performance, at the end of the day they're both spewing files directly down TCP connections so should be about the same.
Many firewalls drop outbound connections which are not to ports 80 or 443 (http & https); some even drop connections to those ports that are not HTTP(S). FTP may or may not be allowed, not to speak of the active/PASV modes.
Also, HTTP/1.1 allows for much better partial requests ("only send from byte 123456 to the end of file"), conditional requests and caching ("only send if content changed/if last-modified-date changed") and content compression (gzip).
HTTP is much easier to use through a proxy.
From my anecdotal evidence, HTTP is easier to make work with dropped/slow/flaky connections; e.g. it is not needed to (re)establish a login session before (re)initiating transfer.
OTOH, HTTP is stateless, so you'd have to do authentication and building a trail of "who did what when" yourself.
The only difference in speed I've noticed is transferring lots of small files: HTTP with pipelining is faster (reduces round-trips, esp. noticeable on high-latency networks)