With DX10 you get better visuals, shadows, reflective surfaces, water, transparency, and so on. A slight increase in available CPU bandwidth is also released, since more work is done within the GPU.
A problem with DX10 is with textures.
Some addon aircraft and scenery items like trucks may not support DX10; check before purchase. Another problem can be seen in many runways, where concrete meets another surface. The texture flutters, like a lamp is switched on and off randomly while aimed at the runway.
Addon airports will avoid that usually, but many of the stock airports exhibit the problem.
If you are OK with that, then DX10 mode will deliver the best view.