I would say that the width of the board is more beneficial to speed composition (because your ennemies have to spread for threat/objectives control) while you can move freely accross the very large table), and the depth of the table is beneficial to range (since you can just camp at 36" and darklight' your way to victory.
Since we have both, it's a matter of opponent : if you have more of a range advantage, take a deep board deployment. If you have more of a speed advantage, take a wide board deployment.
- Playing against T'au, take a Dawn of war. Spread objectives, and watch their less mobile army struggle to hold them, spread, as you can hop from objective to objective and focus their isolated targets.
- Playing against necrons : take a Hammer and Anvil, kill the Tomb blades, jump units and all, and watch them walk painfully slow as you kill all small elite. Then you can gapclose and finish them with rapidfire/cc.