Better chance of the purple/red ones being sweeter IMO. And don't buy the cut ones from the market; buy it whole and do that at home. To choose a dragonfruit,

Better chance of the purple/red ones being sweeter IMO. And don't buy the cut ones from the market; buy it whole and do that at home. To choose a dragonfruit, pick the ones where the "leaves"/"tips" aren't dried out, feel heavy/full and slightly firm.