If this is your first visit, I'd ask whether you want to relax and enjoy a resort vacation or be more on the go.
Surfside has a small town feel and is pleasant and relaxing.
South Beach offers far more activities not requiring lengthy transit.
You can move between these 2 spots with a public bus, Uber or Lyft or a taxi.
Prices and drive time varies, with pubic transit being cheap and slow and taxis being the most $$$.
If you wanted to taxi or Uber from one to the other, that's not an issue at all and should only cost you a couple bucks, maybe $10-15, but you wouldn't walk. Although biking is an option if you're open to the exercise.
The beaches there are nice. There are good restaurants there and in near by Bal Harbour. Travel between the two is easy via uber/lyft, a taxi too but its not that cheap in a taxi. Bus routes are fairly direct but will take twice to three times as long compared to a taxi/uber ride.