Amex MR points (points you get on Explorer/platinum edge etc cards) are currently offering 30% bonus on transfer of MR points to Bonvoy & HH
Usually 3 Membership Rewards points = 2 Marriott points. Minimum transfer amount: 600 points
Simply put, usually 1MR point = 0.66 Marriott Point, with the offer 1MR point = 0.99 Marriott Point
Normal transfer rate for HH is 2 MR points = 1 Honors points. Minimum transfer amount: 2000 points
with the offer 1MR point = 0.65 HHP
As a general guideline its always good to transfer points only if you have upcoming travel plans.
For Marriott, do note they are getting rid of award chart and shifting to dynamic points pricing like HH & IHG's of the world.
Further read on Marriott dynamic pricing -
https://thepointsguy.com/news/marriott-dynamic-pricing/
Spending all my Marriott points this Christmas and not bothering till I understand how bad their dynamic pricing is first.