FWIW - the LHR Ren is a large property so you'll get an exec level room no problem without having to pay. They're good about putting Marriott elites on the exec floor.

The hotel is older than the Marriott & the rooms are small & due for a renovation (although you get the toiletries/slippers/robe benny mentioned above).

But the exec lounge is now open 7days/week with free booze & they put out a HUGE spread of food, both in the morning & in the evening. And the Ren exec lounge has an awesome view of LHR runways.

Unless you're dead set on the Marriott I'd recommend the LHR Ren instead since it sounds like the points are the same.

Cheers.