I am top tier at both.

Some questions to ask.
What are the goals of your brand loyalty? Upgrades? Better treatment? Club access? Free nights?

What is your range of hotels? high end? middle on the road? budget?

What are you looking for out of a hotel? Selection? Price?

SPG is a better program in that you can get status quicker, also by stays. Marriott is nights only.

If you are cashing in points for free nights, SPG has better availability typically but Marriott has more choices. SPG is better typically for international reward travel.

Both have decent branded credit cards, but SPG's adds up quicker.

Marriott has a wider range of hotel brands, offering more choices. SPG has less lower end properties and the ones that have can be awful.

What cities are you going to be traveling to? If the stays rather than the rewards are more important, people could give advice more specifically if we know the location.