Unless you have visited Cartagena previously and have a particular reason to want to stay at Boca Grande, I can't see why you would choose Estelar, Hyatt and IHG over Nacar.

I guess points might play a part.

The beach in Cartagena is grotty. Although Boca Grande is an upmarket neighbourhood I was not impressed with the quality of malls, restaurants and did not think the area particularly appealing.

The Nacar is right in the heart of the picturesque Old Town and has a rooftop pool and bar. It is a fairly small hotel so not crowded and lots of quite good restaurants nearby. A taxi to the beach is a dollar or two. But I doubt one would want to visit more than once.

I am returning to Cartageana in November and am not even contemplating Boca Grande.

In Santa Marta, I stayed in the Marriott Resort which is quite nice but more pricey than the Garden Inn. I stayed in a Garden Inn in Barranquilla and whilst the room was fine I was not that impressed with the breakfast so I went more upmarket with the Marriott.

The Santa Marta Garden Inn is well located downtown as Santa Marta has a pleasant small town atmosphere whilst the Marriott is out of town.

The beaches in Santa Marta are a bit better than Cartagena but are still a bit grotty. If you want to explore downtown I would suggest the Garden Inn but if you want the beach and a little more comfort, the Marriott Resort might be a consideration.