I have never been to NH Krystal, however, my daughter went and loved it. This was about 4 years ago, and I've heard also that it has gone downhill in the past 2 years. When she was there, the staff was wonderful with our granddaughter who was only 1 at the time.
I've stayed in Santa Lucia many times, and am leaving this Thursday for another visit. It is a quieter area than Varadero. There is a dive shop, and also an opportunity for diving with sharks if that is an interest. Good snorkeling, horseback riding on the beach, long beach for walking.
Brisas just reopened after renovations. I've never stayed there, always at Caracol, except when it was closed and I stayed at Gran Club.
Caracol is much smaller, only 150 rooms, very friendly staff.
Happy to answer any questions if I can help.
Cheers,
Beachlover