Quoting Jakob Nielsen here:
" If possible, use radio buttons rather than drop-down menus. Radio buttons have lower cognitive load because they make all options permanently visible so that users can easily compare them. Radio buttons are also easier to operate for users who have difficulty making precise mouse movements. (Limited space might sometimes force you to violate this guideline, but do try to keep choices visible whenever possible.)"
The quote mentioned is point number 8 in the article which you can find here:
Having said that, sometimes both radio buttons and dropdown menus can be reasonable choices. It depends on how the UI is structured and the position of these elements and space constraints. If its necessary to be showing all the choices to the users all the time then radio buttons is the one to choose, else if thats not the case and if you you are expecting the choices to increase in the future then you can go with dropdown menu. Hope this helps.