I want to download the ISO file of the new release of Ubuntu. What is the recommended way to get it? Via torrent or official mirror?
Torrents are a great way to go if you are on a home network and the Internet Service Provider (ISP) does not throttle the service when torrent clients are running (My parents had a wireless ISP that did this; most DSL or Cable services will not). Some Linux/UNIX distro maintainers (I.e. Arch Linux) actually recommend that you download via torrent if possible. This is because your total download speeds will be faster due to the nature of torrents. There is also the additional community benefit of lightening the load on official mirrors.
The important part is to make sure that you download the torrent itself from the distribution's official Web site to ensure that you will be downloading a legitimate copy.
If you are not at a private residence or if the ISP where you are does not allow torrenting, mirror downloads will work just fine too.
When the ISO that want to download has been released a couple of days ago there will be a bunch of people trying to get it from the official server. That is when it's recommended to use a torrent channel, because you're not relaying in only one server your download will be faster than the official server.
None of them is more "safe" than the other one, you should still guarantee the file's integrity. Further information here: Is the Ubuntu direct download exactly the same as the torrent?
Both methods has their pro and con, you have to make the choice.