Is there a difference in stability between prestringing and placing a bead?
There are a lot of pros and cons to both methods of placing beads into your knitting, but when looking only at stability there is a difference between prestrung and placed beads. Having used both methods, I find the placed beads stay precisely where you put them whereas the prestrung beads can slip.

When you place a bead you put it on precisely the stitch required. It can't shift anywhere because the two yarns going through it split and go in different directions. When you knit a prestrung bead into your work, it can travel along the yarn. Generally this means if, for example, you place the bead on the vertical leg of a stitch, it could slip up or down to rest on the horizontal bar of the stitch. This sometimes means it slips to the back of the work. The looser the gauge, the more this problem will occur.