When comparing BitTorrent Surf vs aria2, the Slant community recommends aria2 for most people. In the question“What are the best Torrent clients?” aria2 is ranked 4th while BitTorrent Surf is ranked 19th.
Ranked in these QuestionsQuestion Ranking
Pro Built-in custom search
Built-in search engines allow finding torrents to download right from withing the torrent client. BitTorrent Surf can be customized to include multiple custom search engines.
Pro Free and open source
Pro Supports multiple download protocols
Has support for lots of download protocols including BitTorrent, Metalink, HTTP, HTTPS and FTP.
Pro Small memory footprint
Due to aria2 being a CLI only tool it uses very little memory.
Pro Remote control over simple standartized RPC
XML or JSON-based RPCs can be used to control aria2 remotely with both APIs completely documented. Simplest client that uploads torrent to be downloaded is 3 Python lines long (no 3rd-party libraries, only stdlib).
aria2c --help=#rpc to see more.
Pro Third parties offer a clean web interface
Some web interfaces consist of pure HTML+JS, no server-side needed (except aria2c of course). It uses the same RPC that can work even over WebSockets.
Con Slow development
BitTorrent Surf has been in beta since it was announced in 2013. No changes to the add-on have been made since.
Includes "BitTorrent Bundle recommendations".
Con Unknown underlying technology
It is unclear what BitTorrent protocol features or extensions are supported by the add-on.
Con No native graphical user interface
While there is a web-based GUI, there is no native interface.