<<Please note that this post is valid as of Oct 21st, But is subject to changes due to product improvements >>

There are many comparison posts about Excel vs PBID, But this one should give you a strict answer which one to use in your real life scenario.

Power BI is getting bigger, And bigger. This tool just doesn't stop on red lights.
As an analyst who is used build its solutions and models in Excel and its BI features, You might become very confused.
Excel has been there forever. It has everything you ever needed, And you are very good at it… But Power BI is so sexy ! AHHHHH

So how do i choose ?

First Rule: If there is a doubt – There is no doubt

This famous phrase from the army works here as well 🙂
Excel BI Features (Power Query and PowerPivot) can be imported into Power BI desktop , But the other way around is not possible. Yet.
So, If you start with Excel and then want to move later – You are good, But if you start from Power BI desktop you cant (well you can manually but it requires to build everything from scratch)

When comparing them we need to remember Excel has 2 active versions these days: 2013 and 2016

There are many reasons to use Excel to build the model  (VBA,Cube formulas, Sets, Power Map,Linked tables, Excel formulas which do not exists in Power BI, and more…),  So we are going to focus on the reasons to build it in PBID. And in the rest of the cases – start from Excel.
Because you can push an Excel workbook containing Power Query queries and a Data model straight to the service , I will go through just the cases where i think you should create the whole solution inside Power BI desktop:

If you need to use Tabular 2016 features

Power BI desktop is a fast pace moving tool, So Microsoft decided to use it as the first tool to contain the 2016 version of the SSAS tabular model.
This gives us many new features like Many 2 many, New DAX functions, Variables in DAX , Performance and more.
If you are using Excel 2013 – You are missing those powerful features, And if you are already in Excel 2016 There is still some missing  (like many 2 many)

If you want your Reports packaged inside the solution

When you upload an Excel workbook and choose to import the BI features, You will need to create the reports in the service.
But if you ever need to move the solution, Copy it to another account, Or even use it as a starting point on another project, This will be impossible to do if you started from Excel, I mean, You will have to create all PBI reports from scratch.

If you need to import data from "exotic" sources

The Excel version of  Power Query is a bit behind the PBID version due to obvious reasons. Some data sources are NOT supported yet in the Excel PQ:

This is the best practice we are working according to right now, I would love some comments about how you choose your tool