The 15th have a focus on combined arms warfare, with tanks, APC's, Light Armoured Recon etc.
The 506 IR have a primary focus on infantry combat. We have some support assets such as fixed-wing and rotary airframes and bradleys but these assets are used more sparingly.
In all honesty, they're both good units. If you're looking for realism you can't go wrong with either choice. Despite the 506's more focused approach, I still enjoy my time as a support asset.
There are minor differences, we have a policy where fun in some circumstances is taken as priority over 100% realism but it honestly comes down to whenever you think the USMC or the Army is 'cooler'.