Azure SQL DB vs Azure managed instance We are in the early stages of planning to move some of our on-premise database to cloud and wanted to go the PaaS way.  I understand that managed instance is in preview.  

From what I have read, ignoring the cost, is there any reason why not to go with managed instance? Does it not make the migration easier while still being PaaS/Managed so that we do not have maintain it?  Looks like the best of both world to me.

The only question that I couldn't find is how we could handle disaster recovery for managed instance?  With Azure SQL DB we can have Geo-replication in another region, but couldnt find anything similar documented for managed instance.

In the initial stages we are planning to move an OLTP application which needs to be highly available and have very low latency queries.
It really does come down to what you need and why you need it. Managed Instance has a single, huge, strength over Azure SQL Database, cross-database queries. If you need to query between databases and you don't want to rewrite the code, then Managed Instance is the way to go. There are some other advantages such as having SQL Agent (easily replaced with Azure Automation) and some other behaviors, but the cross-database query is the huge driver. 

I can't answer the geo-replication question because I haven't tested that from Managed Instance, so I'm not aware of the level of support there.