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Want to check if we have any guidelines (metrics) helpful to decide either Mongo or SQL will be good fit while Upgrading to Sitecore 9, especially if the current version in below 7.5 (No Mongo). Mentioning few criteria below, can be extended further:
Neither Sitecore 9 nor Sitecore 9 Update 1 support Mongo for XConnect. Your only choice is SQL Server or Azure SQL. As you use Sitecore 7.2 I would definitely go and migrate to as of now latest version - Sitecore 9 Update 1. I would say it is even better as you don't need to introduce Mongo DB in your landscape and can work only with SQL Server as client is already used to with Sitecore 7.2. I would definetely contact Sitecore Sales representative to find help for you. Take a look on Express Migration Tool 3.1 -> https://dev.sitecore.net/Downloads/Express_Migration_Tool/31/Express_Migration_Tool_31.aspx.
If components in solution are still Webforms based, I would definitely consider converting them to MVC as part of this migration process or at least as next step. Maybe not as big bang (all at ones) but one by one.
Also use nuget packages in project instead of referencing dlls in lib folder as I would expect is used in solution. This was common thing back in those days :).
As MongoDB is not supported by Sitecore 9 there is no recommendation yet. Any advice you get will only be a speculation.
Having said that, I think that there will not be any significant differences in supported topologies, performance or features (sharding, encryption, etc.). So, I would compare license and maintenance cost as well as evaluate development skills that your team possesses for both of these technologies.
If anyone has more insights, I am very curious to hear.
