RSK Side chain vs Velvet fork

Greetings, so there has been a lot of conversation about the possibility of cross chain communication with Cardano and other similar logic smart contract networks via a velvet fork and rightfully so. But I have not seen much discussion around what appears to be an alternative to Velvet, being Rootstock side chains, looking to gain some perspective from everyone.

yes Litecoin and Cardano continue to talk partnership over a Velvet Fork

So as far as velvet implementation for smart contracts and interoperability or RSK chains for smart contracts?