SLT can be controlled out of SDI with the upcoming release. The main questions is where you want to setup your central replication component and which other use cases you want to serve.
We have a lot of customer started with SLT->HANA and afterwards added a scenario like SLT->BW or Central Finance w S4Hana.
When you think about this and add SDI. You will have to data streams required. SLT->BW/Central Finance w S4Hana and (SLT->)SDI->HANA. Only SLT can directly talk to BW or Central Finance/S4Hana.
Best