I was looking around and found people suggesting that pentel fude is the best brush pen to practice kanji and calligraphy. I ordered this extra fine one but then I found videos where they use medium tip. Will this make much of a difference or should I buy the medium one?
Correct writing of Characters is important for readability of the written Language, and the precise description of the Fude Pen in the Question is important for replying:
The Fine size Fude Pen described is excellently suited for writing ~0.7cm through ~1.2cm characters, and can be used for writing characters outside that range to some extent, and the Medium size is excellently suited for writing ~1.0cm through ~1.8cm characters and can be used for writing characters outside that range. For characters less than ~0.6cm the Medium requires much skill to write characters nicely, especially when writing Kanji having many small/ closely spaced strokes.
The tips are quite precise and stable on both of the Fude Pens mentioned in the question, which is part of why they can be used for effectively writing such ranges; the Medium will provide thicker strokes, eg ~0.2cm, more readily than the Fine size will, and the Fine size is more suited for making tiny strokes and little diacritics, and showing stroke tip types for smaller characters. The Medium is excellent for Greeting Card Headings and Holiday Greetings, and the Fine is more excellent for smaller writing of addresses and smaller text on standard size New Year Cards and other various Postcards. Because of the size, the Medium applies more ink with the same amount of effort.
Which Fude Pen to ideally use depends somewhat on the sizes of the characters being written, and also the style of the writer: the above are general suggestions for conveniently writing basic standardly proportioned characters. The dimensions suggested are fairly accurate however, as they are from recent comparisons and specific measures during actual Fude Pen comparisons at a large premier writing supplies place by several native Professional Language and Language Writing Sensei.
The weight and balance of the Fude Pen gradually changes noticeably as the ink is used. Both sizes use the same ink refills, so if eventually want to also have a Medium size, they use the same ink refills. The Fine size and Medium size are both ok for beginner and advanced, the Fine size especially for writing ~1.0cm or smaller Kanji and Hiragana. It is important to keep the cap on when it isnot being used, and good to keep them from going dry without immediately replacing the ink refills.
Those Fude Pens are popular with some very skilled writers, can produce high quality results, and are convenient for doing much writing, as the tips are very stable and the ink flows somewhat automatically.