If you have a right arm off spinner bowling to a right hand batsman is it better to have a close in fielder on the leg side or offside and what are the determining factors?

Though I was a leg-spinner, I ALWAYS wanted a short extra-cover AND a short mid-wicket. I imagine if I was bowling offies, that would not change, as batsmen beaten in the air or mistiming a drive would be caught. Also, to save a single off a defensive push.

If you are spinning the ball from off to leg the first option should be for close in fielders on the on side. The batsman is more likely to get an inside edge onto his pads and pop up on the onside.
I used to play with Bobby Aitken who would field that close at silly mid on that he would try and dive forward and catch the ball just after it came off the face of the bat. I remember one time during a team discussion someone asked him why he fielded so close. He answered that you were less likely to get hurt close in because the ball hadn't had time to build up speed. He wasn't a physicist.