Stevia is a better option because it is entirely plant based. Synthetically produced artificial sweeteners like
sugar free have been shown to produce side effects on the long run. But keep in mind this is a situation of the lesser evil. Even though they are calorie free/less but they still are evil.
Next Steps
Reduce sugars and follow your doctors advice.
Health Tips
Keep a chocolate or something rich in calories with yourself because Metformin is known to cause hypoglycaemia on occasion. It is essential to maintain a regular diet pattern.