Front Row (L->R): Vibe, Marimba, Marimba, Marimba, Vibe
Back Row (L->R): Bells, Synth, Malletstation
I tend to lean towards these types of setups for the added ability to make it visually appealing and symmetrical. The last thing I want from my front ensemble is visual noise.
From a musical standpoint, centering the marimbas - usually the stronger players - will put them in the position to be the pulse center for the others. Also, when hearing the amplification OR acoustic sound - the visual look of the ensemble reinforces the auditory experience.