We like to think that all patterns that we carry are the “best”. The pattern to choose, however, is the one that you like the most. Some people, for example, will pick a liner that matches the theme in their back yard.
Each gauge has its own advantages. Your dealer should be able to discuss this with you in greater detail.
Although it will be subtle, yes you will. It is the pattern you choose that will reduce the seam visibility. Please contact your pool dealer to discuss the available pattern choices.
The change in temperature between the choice of a dark liner and a light colored liner is insignificant. There are too many variables that can determine the temperature of your pool. For example, your home location, do you live in the northern part of the nation or the southern. The pool location is it surrounded by trees, or in a sunny spot? The best choice is to buy what you like.