Soften always has a muted "T" and Often does only 3/4 of the time.
You can use whichever form you prefer, and which makes your phrase easier to listen to, keeping in mind that highly educated British people say the "T" slightly more frequently, and often unconsciously employ both. The "T" is inconvenient for fast speaking by posh people, so it is often omitted, and it's regionally and culturally variant.
"Oft" is also a word, and it's the archaic form of often. the addition of the "en" at the end has made the word a bit too long and over-articulated. The reason that the "T" is muted is because the archaic spelling is over-complicated
I went to a fairly good school in East Oxford, where the Harry Potter actors were sourced from because of the "good English pronunciation" there, and I have to be honest, I have never noticed that people say both forms of often! It's news to me! because both forms are said very commonly and with relatively little importance. 25% of people say it with "T", and many people say both depending on how fast they are speaking. So my point is... It really doesn't matter. If you ask me, I would have thought that the proper way is "Often", and that not that many people say "Offen"... So it depends your region and social entourage.
Well received English speaking people probably think the same as me, that "ofen" is informal, convenient, vulgar form of the word, and a bit more common in Scotland and America, and ofTen is the received English like you would hear on the BBC and radio... In Scottish and American, slightly more emphasis is placed on the first letter "O", which makes articulation of the T a bit slow and tedious, and in "harry potter English" the "O" is less emphasized, where as the "T" provides emphasis on the word if necessary. If the word is an important part of the phrase, or you want to make sure you want to be heard, including the "T" is probably a good idea.