Lars is without a doubt a more well rounded midfielder, a player without any glaring weaknesses. Sven´s shooting technique is not really impressive and while his passing game is solid, it lacks the creativity and range of the one of other top class midfielder.
However, this does not make Lars the better midfielder, just a more versatile one.
Sven is a very specific type of a defender: a classic destroyer. A player with pretty much exclusively defensive strengths. He is an absolute stamina beast, having an enourmous work rate. He is also fantastic in anticipating passing and running routes, which make him Dortmund´s best pressing player. On top of that he is as good in tackling as pretty much every CB in the world, while being mobile enough to play in the DM.
When it comes to deciding between both twins, it comes down to what your team need. Do you want a midfield all rounder, who is more flexible in being utilised you go for Lars. Do you have the specific role as destroyer, who allows the CM partner huge freedom going forward, in mind you go for Sven.
There is one thing to keep in mind, though. There are several midfield all rounder comparable to Lars Bender around, while there is only a very small amount of destroyers on the level of Sven Bender.
It is kinda like choosing between a jack of all trades and a master of one thing.