We deal with a similar issue. However, what makes it interestingly complex is that our branch and audience is B2B.
Besides being very important to know which variations has more traffic volume, it seems that a single/plural variation can have in the background different queries' needs or origins.
For example between "payment services" and "payment service", there seems to be no difference and of course as the previous mozers comment, Google is intelligent enough to present results regardless of the spelling.
However, "payment service" could be (and tends to be) a B2C query. Usually referring to a specific service, i.e. "jc penney payment service". That is of course not relevant for us.
In conclusion, I would recommend to also have a look at the content's type and context that the different variations trigger.
