IMHO...the felted join is most invisible, and no bulk added to the joined area. The russian join can still be a little fatter in the affected area.
Not all yarns will 'felt' because of the fiber content...eg cotton. However, if that be the case, the russian join is your other next best option.
I use the felted joins wherever and whenever possible!
Some folks call the felted join the "spit and splice" Ick! I won't use spit!
I keep a little shot glass of water with me!