Delta has the points exchange with SPG too. You get to double dip essentially.
Money spent at either gets points for the other.
Buy a $500 ticket on Delta, get 500 Starwood points.
Spend $500 on a room at a Starwood, get 500 DL Skymiles.

SPG is better at redemption IMO. I do like the late checkout option.
Premium lounges tend to be better at Marriott.

It really just depends what he wants out of it all.

If SPG had more and a wider ranges of properties, I would think it would be a no brainer to go with SPG. But that is just not always the case.