You probably got that information from the Wiki: http://www.mopedarmy.com/wiki/Motori_Minarelli
Here is a NGK Spark Plug parts labeling key that will help you understand what the part number means, and the physical difference between NGK B5HS and B6HS: http://www.ngksparkplugs.com/docs/tech/design_symbols_plugs.pdf
From the above PDF, you can see that literally the only difference between the two plugs is the "Heat Rating" 6 is rated "Colder" than 5.
What's "Heat Range" or "Heat Rating"?
Read Here: http://en.wikipedia.org/wiki/Spark_plug#Heat_range
The 5 conducts heat away from the tip less efficiently than the 6, so the 5 operates better when the rest of the bike is not as hot as it could be.
Buy a few of both, and don't over-think it, it's a spark plug and the V1 is known to be very reliable.