It all depends on what your goals are. If you are trying to accumulate the most points, then the SPG Amex will yield you 6 points for every $1 spent at Marriott hotels after you transfer them from your SPG account (2 SPG Points X 3). However, these points will not count toward your elite status or your lifetime status because they do not count as earned Marriott points. When using the Marriott Visa, you earn 5 points for every $1 at Marriott hotels (plus a % bonus depending on your current status) and they count towards your elite status and lifetime status. Additionally, every $3,000 spent on the Marriott Visa gives you 1 Elite Night Credit, which you will not earn through the SPG Amex. Considering that eventually, everything will be combined (estimated sometime in 2018) and will all be Marriott, working towards your status is worth earning 1 point less, in my opinion. So unless your main/only goal is to rack up as many points as possible to use for redemptions, I would suggest using your Marriott Visa at all Marriott and SPG locations and using your SPG AMEX for all non-bonus spending categories to earn more supplemental points until 2018 when everything combines together.
My partner and I use the Marriott Visa as our shared card and all of our household and other shared expenses go on it. Then we each have our own personal cards (his is the Chase Sapphire Reserve and mine is now the SPG AMEX).