If you're looking for a gaming mouse, go with the G400.
If you're looking for something general-purpose, the G400 is bigger, but the M705 has a slightly more ergonomic shape, so it depends on the size of your hands. The M705 also has the free-spinning scroll wheel which can be very useful, and it has a tilt wheel which the G400 doesn't have. One isn't necessarily better than the other; it's a matter of preference and which one you think would suit you better.