Recs.
Updated
Deluge is free, open source and cross-platform BitTorrent client. Deluge aims to be a lightweight, secure, and feature-rich client.
SpecsUpdate
Pros
Pro Full-featured
Deluge covers basics such as Prioritization, Selective Downloading, DHT, PEX, LPD and Magnet URIs. It includes MSE/PE for security. It has graphical, web and command line interfaces. Has UPnP, NAT-PMP, NAT traversal for automatic router configuration. Has UDP tracker data transfer protocol support. Even covers IPv6 and proxy servers.
Pro Portable
µTorrent can be run without having to be installed. It can be put on an external storage device (such as a USB drive) and run directly from it. A portable version of Deluge can be downloaded from SourceForge.
Pro Utorrent compatible
You can download using both clients µTorrent and Deluge on any supported operating system.
Cons
Con No super-seeding support out of the box
Super-seeding is an algorithm used to allow downloaders become uploaders quicker. Deluge lacks support for this functionality out of the box. However, the ltconfig plugin can enable this setting: http://forum.deluge-torrent.org/viewtopic.php?f=9&t=42887.
Con Files can't be downloaded sequentially by default
Sequential downloading can be used to stream media files such as video and audio while the download is still in progress. By default, Deluge requires waiting until the complete file is downloaded before it's ready for use.
But it should be mentioned that there exists a plugin: https://github.com/JohnDoee/deluge-streaming for video streaming from Deluge.
Con Demon not lightweight as expected
While it has been improved a lot in the last years, when it's used in "headless" mode, the python demon still tends to use more resources than expected. It's noticeable especially on devices like the raspberry, and for instance, rtorrent and transmission use much less CPU.
Con No web seeding suport
Web seeding support allows downloading file parts from an HTTP source alongside the swarm. It's usually used for long-term seeding and easing some of the load on web hosts that supply direct downloads by offering a BitTorrent download as an alternative while still ensuring that the file is always available in its entirety regardless of the swarm. Deluge lacks this functionality.
Con Keeps Unwanted Files (Poor selective Download)
Deluge creates all the files even those that you marked "not download". Making your directories messy.
This is very bad when you want to create a drive specific for large files and you are unable to delete or move small and unwanted files, such as a video partition with large allocation unit.
[qbittorrent doesn't do that]
Con Can't "force start" downloads
There's no way to manually override settings for download queuing that's available in other torrent clients. For example, if there are 2 files in the download list and the software is configured to queue just one at a time there's no way to force a download of both without changing the settings. A workaround plugin can be found here.
Con No built-in broadcatching support
Broadcatching is a method of automatically downloading content as soon as it's available. Broadcatching in BitTorrent clients is usually implemented via RSS. Deluge requires manually selecting each download by default. Fortunately, there are multiple plugins available that add this functionality with the possible caveat of plugins not having the same quality standard as the client itself.
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Flagged Pros + Cons
Pro Utorrent Compatible
You can download using both clients µTorrent and Deluge on any supported operating system