I own both, Watanabe Kasumi 240, Tanaka 270, and Watanabe 270. All different knives, very different actually. The Tanaka is the lightest of the bunch by a long shot, spine is comparable but it's ground A LOT thinner, cuts great (the more silent cutter of the three) but release and power isn't as good as Watanabe. Steel I am personally a sucker for the Watanabe (or Toyama since it's the same thing lol) but the Tanaka B1 is no such by any means.
The tip of the Tanaka is much thinner than the Watanabe and does better on onions OOTB. Profile on Tanaka has a longer flat spot and higher tip while Watanabe has a lower tip and subtle curve.
The Watanabes might not be the most silent cutters (especially the Kasumi) but the blast the produce without mercy, and some tweaking brings them to the next level (thin the tip, and for my Kasumi I raised the convexity's apex a couple mm, the KU has a nice cutting grind but chubby tip).
The Wats are heavy as axes so keep that in mind. Depends on what you like, I prefer the power and all around feel of the Watanabes personally.