The SIP can be looked at as a tool for short to medium term incentives for those employees who are members of the SIP, whilst the EBT is generally established for long term ownership stability. Unfortunately payments to an EBT by a company no longer attract tax relief unlike payments to a SIP.
Whilst most employee owned companies will have an EBT it is the larger companies which might have an EBT and a SIP working together. Other share ownership schemes exist (both approved and unapproved) which can be used in combination with each other and the EBT.