Verdant describes the land; whereas lush describes the quality of the vegetation growing on the land.

"Verdant" comes from the Latin viridis which means "green". You can describe a piece of land or ground as "verdant" if it is green from the vegetation (ie grass) growing on it.

But "lush" describes the quality of the vegetation itself - that it is luxurious, abundant, flourishing. You couldn't really say that about the ground itself. Likewise, you wouldn't describe grass as "verdant" simply to say that it was green. You would just say that the grass is green! Verdant doesn't simply mean the colour green - it specifically describes land that is covered with grass/vegetation.

Verdant describes the land; whereas lush describes the quality of the vegetation growing on the land.

"Verdant" comes from the Latin viridis which means "green". You can describe a piece of land or ground as "verdant" if it is green from the vegetation (ie grass) growing on it.

But "lush" describes the quality of the vegetation itself - that it is luxurious, abundant, flourishing.

This ngram is interesting - it seems that in the past it was more common to describe a valley as "verdant", but in more recent times there is a trend to describe a valley as "lush". However these are very small numbers, so I wouldn't place too much emphasis on it.

I would say that either of your suggested options are fine and convey the same meaning. "Verdant" might not be as widely used or known, but, also "lush" has taken on a secondary, more informal use in modern British English. Saying "lush green valleys" though makes it very clear you are describing the grass and vegetation.