The short answer is that you can't compare film *specs* and decide
what film to shoot.  There's a lot of personal subjectivity involved.
But I don't think you need to shoot a "bazillion" rolls, either.  More
like a half dozen.

The most meaningful grain specification is Kodak's Print Grain Index
(PGI) because it is tied to what the average observer actually sees in
a print instead of what is in the negative.  More importantly, PGI
value differences can be related to "just noticeable differences" in
grain.  The downside is that you can only do this for Kodak film.
(Attempts to convert RMS granularity to PGI or vice versa are a waste
of time because they measure two different things.)

So my first suggestion would be to correlate your eyes with those of
the "average observer" by shooting similar scenes on a roll each of
400UC and 100UC  that have about the same expiration date and have
them processed at the same time in the same lab.  Kodak says these two
films differ by nine PGI or about two times a just noticeable
difference.   Now you have some sense of that grain difference,
keeping in mind that PGI (or RMS, for that matter) is expressed in
terms of a gray color – not necessarily any particular pure color.
Maybe one shot of a gray card would be useful.

If you want to compare across manfacturers, then you would add those
corresponding films, again keeping expiration dates similar.  Now
however, the choice of processing becomes more difficult.  You could
choose to put all the films through your favorite lab together, but
the argument against this is that the lab might favor the products of
a particular film manufacturer.  The alternative is to find a lab for
each film manufacturer.  This is your call.

Finally, I will tell you that experiments to find the best grain are a
largely a waste of time.  Most photographers are aware that higher
speed usually means higher grain.  What is less well understood is
that grain is also intimately tied to sharpness, color saturation,
color hue errors, and several other design considerations.  All these
things can be traded off against one another, and most do not have
published metrics associated with them.  The top tier film
manufacturers have about the same technical capability, but they may
choose to trade these various film features in different ways.  It is
not simply a matter of who has the best grain at any given speed, but
rather it's who has the best *combination* of features that *you*
prefer.  So in the end, you have to shoot some rolls of the things you
like to photograph.  I would just advise you to minimize the
extraneous variables, e.g., expiration date, scene choice, processing,
as much as you can.