If the scene fits in card memory... 6, 8, or 11 GB won't matter.
11GB is better - you can handle bigger scenes without needing Redshift to use out of core memory - which will slow things down - but unless you're dealing with huge geometry and multiple massive texture maps an 8GB card will do fine.
I recently rendered 8 million instances (circa 25 billion polys) on my 3x 1070 Ti's (8GB) without a problem ; )