With these 2 directly opposite one another I suspect for many of us the choice between the 2 will become one of price, but it's important to compare apples to apples. Hopefully a side-by-side type comparison will help weigh up the value.
Kensington Autograph or Marriott
Autograph Xenia has 8 room types
Xenia has no Exec lounge
Petit single 97sqft
Petit double 118sqft
Classic double/twin 140sqft
Terrace double/twin161sqft
Exec King/Twin 183sqft
Exec balcony King/Twin 194sqft
Deluxe King/Twin 215sqft
Suite balcony King 592sqft
Marriott has 5 room types
Marriott has interior exec lounge
Deluxe Queen 258sqft
Exec King/Twin 258sqft
Exec large King 323sqft
Junior Suite 2x King 516sqft
Kensington Suite King 516sqft
It's worth remembering that Marriott has 2 types of London properties bearing the Marriott name, the 5-stars at Grosvenor Square, Park Lane, County Hall and West India Quay and the more brand-typical 4-stars at Marble Arch, Kensington, Regents Park and Maida Vale.
In my experience the Kensington Marriott either doesn't upgrade at all, or when it does only to an Exec King. Hence even as a Gold/Plat your room is likely to be 258sqft, though I am very, very sceptical about that size - I'm not sure how they've measured it but my room at the Marriott has always felt pretty small, nothing like 250sqft. I personally felt it was somewhat less than 200sqft. Even so, the Marriott room size isn't too bad for London but a lot smaller than its 5-star properties. The Xenia hasn't been open long enough to be sure of its policy but so far it would seem it does actively upgrade but maybe only by one category. So Gold/Plat reserving a 97sqft single can, subject to availability expect a 118sqft double. Booking a 118sqft double should see you into a 140sqft classic.