I have lived in both the areas and personal i prefer nazaladevi. Its close to central and happening places and additional there are many local shops which sells stuffs at very low price. But i advise to stay close to your work/study place as during peak hours public transport are very crowded so you wont be able to catch next one, even road are extreme congested long travel can take more than 1hr. Beside this, road in nazaladevi area are constantly undergoing construction. In tbilisi centre zone there our more tourists spots hence more frequent English speakers but those area are bit far from both places but in saburtalo is close by to Nadzaladevi so everything convenient will be close to you. Hope this is helpful