In practice, there are more factors than just how quickly one's application gets reviewed.
One may get through the initial steps faster, but it does not mean that the overall process will be smooth. Individual circumstances and who processes the file matter.
You want to remove undue obstacles by getting an application in the system sooner.
Again, highly theoretical. Everyone can decide for themselves. Perhaps it is 'smoother' to apply online first - but not yet shown to be faster, overall.
But MANY who filed spousal online apps are quite unhappy that the first stage is taking quite a bit longer.
I've no opinion really on which (online or paper) is 'better' for each individual. There are advantages to applying online (particularly for those who have original documents in different countries and sending them back and forth is time consuming and expensive).
But it is absolutely not yet confirmed - in practice - that one is
faster than the other.
It should be theoretically and probably will be eventually, but not shown to be the case yet.