We were uncertain re: Old Town or the newer part when we planned our trip a few years ago. But, we chose Old Town, and then we were so pleased once we arrived (saw the difference). We found the Old Town section more charming, easier to stroll, and it met our needs perfectly. I cannot recall where we had dinner (early), but it was a very small casual place, but with excellent food and service.
We did walk down to the new section of Marbella to see all the fancy yachts and high-end shops, but after strolling for a few hours, we were comfortable to return to the Old Town section, which we found much quieter and intimate in feeling. We are also not night owls.
We were there in early March, so not yet beach weather, but pleasant, light-jacket weather.