If you are not an SPG Plat already, there are apparently some challenges you can do to qualify pretty quickly.

75 nights is probably more of a sweet spot for SPG, with Your 24-hours at 75, 10 x Suite Night Award at 50 nights. I don't really see the value in SPG 100 as Land-of-Miles mentioned above.

Of course SPG Plat now comes with Marriott Plat so there is that benefit.

Lifetime status can be good, although with SPG I am not sure how long the scheme will run independently so unless you've been a Plat already, you might not get to meet "10 years as a Plat and 500 nights" criteria.

In terms of SPG upgrades, it varies a bit depending on the property but by and large I find the SPG Platinum upgrades pretty solid. I've had anything from just a normal upgraded room all the way through to presidential suite with SPG, whereas I've only had a presidential suite type of upgrade from IC properties that I am a regular at, and it wasn't an RA benefit but a frequent stayer benefit that the individual properties decided to give to me. With SPG it's not limited to two categories either. Being able to know for sure in advance with Suite Night Award x 10 is also nice.

Club access is guaranteed for any Plat where there is one, and in the absence of that, you tend to get a full breakfast in the restaurant. Platinum welcome amenity points (should you choose the points) is better than the IC with 500 SPG pts (vs 600 IHG points). Reward nights still attracts the Club access.

Nothing is opaque with SPG which makes a nice change from RA.

Apart from the minibar, I don't really see much difference between SPG Plat and RA. Further, SPG bonuses aren't tailored in an annoying way and it's very solid e.g. current 1,000 extra points per stay, although there are quite a few non-participating properties by the looks of it.

Some SPG Club lounges are mediocre at best, although many seem to be improving them.

So, if you need to choose either RA or SPG, rather than both, then provided there are enough SPG/Marriott properties in the areas you will be frequenting, I'd be inclined to go for SPG personally, and then use both SPG and Marriott.

I have one fewer reason to stay at one of the ICs - the Sheraton nearby has been refurbished and the Club that used to be extremely poor has been improved a lot, although the rooms remain somewhat worse than the IC. The rates are very similar, or Sheraton is a bit cheaper, and it is in a more convenient location than the IC. That becomes a no brainer choice if they don't renew my RA.