i have four of these- one factory 7900, two 6401-7901 converts, and one 6401-BB convert. i can't tell any difference in them. the OEM cylinders are def better. the first BB kit was returned for quality issues. i bought the BB about 7 months ago, so it is possible that they are higher quality, though i haven't heard anyone praising their quality vs OEM in the way that the 066BB earns high marks in terms of quality vs OEM. if i will ever buy another of these saws, it will receive the OEM cylinder. the extra $60 is money well spent.
note- for anyone who does use the BB, please check the squish. mine was in excess of 0.040" and i have seen others report similar measurements. i used some gasket paper to bring the squish down.