Which type of fan is better for wind tunnel testing?
You could make either one work, especially for a "straight  through" tunnel.  The key is to smooth out the turbulence after the fans to as straight and undisturbed a line as possible.

Smoke trails help check your wind stream before the model is put into place for testing, 
and remain valuable to see changes in the wind flow pattern as it passes over the forms.

There is an excellent article on the internet:  howstuffworks wind tunnel, and a very good diagram showing post fan stages such as stators, vanes, diffusers, and settling chamber that help create a desirable flow.

For testing of lower subsonic designs, a straight through tunnel, such as the one the Wright brothers built, may be easiest to construct.