Singular and Plural form

Doesn't appear to be a difference in meaning. Really seems like its more so a difference in what sounds better in a sentence.

For example:

(a) Since you are a loyal GOLD VISA rewards member, you have more varieties of choice in monthly perks.

(b) Since you are a GOLD loyal VISA rewards member, you have more of a variety of choices in monthly perks.

I think sentence b is more of a smoother sentence than sentence a.

These two phrases are different. For the everyday situation where there are many options in front of you, "variety of choices" is standard and idiomatic. There are a couple of ways "varieties of choice" could be interpreted:

1. As an indication that the options being presented have already been narrowed down based on some quality criteria or preference. For example, "Of all the varieties of liqueur, Fernet and St-Germaine are my varieties of choice". Note that this uses "variety" to mean "a specific type of something (when other types exist)" as opposed "multiple types of something" (definition 4 and 3, respectively)
2. As a description of multiple facets of the idea of choice (decision-making) itself. For example, see this academic article, which describes the spiritual and the political as two varieties of human choice.
3. As a poetic, nonstandard phrasing of "variety of choices", used for literary effect.