SDCCH hop in any case for BBH as all TS other than the BCCH i.eTS-0 TRX-0 are hopping.
For SDCCH in FHS simply they dont hop on BCCH and hop on hopping radio.
As far as the agvantages of having them on hopping or not hopping,lest focus on SFH. the issue narrows down to the noise floor if the BCCH is cleaner then SDR is reduced by keeping SDCCH on BCCH radio which is normally the case and is the preferred scenario. for the hopping frequencies being cleaner SDCCH can be kept on hopping radios too.