MongoDB will introduce new element into infrastructure of your solution. This increases cost of servers, cost of maintenance and at the end complicates the overall solution landscape.
In scaled solution you would need couple of Mongo DB server e.g. replica sets to support failover, redundancy and data availability.
If you stay with MS SQL Server installed on VM in Azure or on prem, it would be just couple of more databases at the end hosted in your already existing database server where you host master, core, web and other Sitecore databases. On the other hand, if you use Azure DBs this could be really cost solution in oppose using Mongo DB.
2) Tooling and Debugging
You also need more tooling like RoboMongo to query data for debugging purposes and for developers need to know how to query MongoDB as it is way different in oppose to classic SQL Server.
For administrators new set of monitoring tools / scripts to handle these servers.
If you stay with MS SQL Server, you don't need to install additional tools and learn new queries as you are good to go with MS SQL Server Management Studio (SSMS).
You are getting the data from analytics index so whether you are using MongoDB or SQL Server, your decision won't significantly increase / decrease performance of reading data in your solution. Only question of performance here how fast can data be indexed using either Mongo DB / MS SQL Server. This is I think marginal question here. I would believe that this is a bit quicker for MS SQL Server as this is already third version of product which uses it. For Mongo DB querying data might be slower as it could be not so optimized in API (first version caution see below).
4) First version caution
Sitecore 8 was using Mongo DB to hold Analytics data already. Sitecore 9 Initial Release and Update 1 were not supporting Mongo only SQL Server. Sitecore 9 Update 2 is the first version of Sitecore 9 where MongoDB support was introduced once again. It is also disabled by default. I would wait for next update or Sitecore 9.1 to experiment with this.
5) Pros for Mongo DB
On the other hand, I believe MongoDB should be faster processing unstructured data than MS SQL Server although Microsoft made progress processing json data in recent versions of SQL Server like json native data type and so on.
Usage of MongoDB would be great for existing projects that are on Sitecore 8 and want to migrate to Sitecore 9 using existing environments.
If you use Azure DB then Mongo DB would be relatively cheaper solution. Depends on architecture and landscape that you plan to use.
To summarize, for your greenfield project, I would probably stick with MS SQL Server and wait for some fixes in upcoming releases regarding MongoDB support so you are not "early adopter" :) Then evaluate and maybe make a switch.