Torrents are usually faster than single server file transfer.
And, as far as downloading a modified .iso, you have to trust the source (uploader) -- read the comments, check how many seeders, etc. Sort of like buying on eBay. You will always get one or two (its a viris no wrk waaaaa) comments, but if it's a trend and/or a relatively new uploader, beware. If it's a trusted uploader with multiple successful uploads and 300,000 seeders, and 99% positive posts, it's fine.
However, for those paranoid, you can always find an uploader with an unmodified MS .iso -- which is useful if you lost or scratched your install medium and wish for a fast download compared with MS servers. (Although, using download managers capable of downloading multiple file segments from the same server, it's possible to approach torrent speeds if the MS server load isn't too great).