It often depends on the format
Normally, in singles, Leftovers is preferred. Generally Bulky Tanks and Walls are the users of Leftovers (or variants like Black Sludge) like Heatran, and as they will most likely survive for a large period of the match, the overall recovery Leftovers provides is higher than the single use of Sitrus Berry. However, there are exceptions including Harvest Tropius, Exceggutor and Belly Drum Azumarill.
In doubles, Sitrus Berry is almost always favored over Leftovers. This is due to the fast paced nature of the Doubles and VGC metagames, so the higher immediate recovery provided by Sitrus Berry is more integral than the gradual increase provided by Leftovers.
