I got a Rotadent because my periodontist recommended it. I used the same brush for 9 months and then had a checkup. She said to replace the brush head every 3 months because the filaments will degrade and become rougher, causing them to be more abrasive to my gums.
She also said that she suggested using antiseptic mouthwash with the brush to get at the bacteria under the gums.
Anyway, it seems Zila provides the lifetime guarantee because you have to replace the brushes (recommended 4 times per year which would be about $50, from
Zila). Seems reasonable to me, except that they have to prevent their customers from finding competitors willing to make and sell replacement brushes for less, plus the "lifetime guarantee" is only good as long as there's a company there to honor it.
They come in packs of three for $12 each, or one at a time for $15, from the site above.
I haven't used the Oral-B.