Both are first class monitor manufacturers - it's a case of which offers the best specification at the price you wish to pay.
Like Sir Radfordin I've had superb after sales service from iiyama.
It's a long story in that dabs supplied me with two new iiyama S702GT monitors three or four years back. One was clearly a return and even had the return note in the box.
I didn't bother with dabs, but contacted iiyama direct. The company was horrified and immediately swung into action.
A brand new (manufactured only a month earlier) monitor was delivered next morning and the dabs supplied one collected at the same time.
In the case of Mitsubishi, many of its larger monitors are also, or have been, rebadged as Sony, Dell etc.
However, whichever model you choose, make sure it is capable of at least 85MHz refresh rate at the maximum resolution.