I am trying to give you an answer as I see that nobody answered your question until now. I am learning too and so anybody may correct me if I am wrong. I Thing the car that you rent is expressing that you not yet rented the car, you will rent it in the future. The DIG-form can only be used for non-future -sentences. That means in this case you would have to use future tense or the construction with "ICEGI"
It is redundant, possessive adjectives are often only added for emphasis. For example if the previous sentence was "Bizim kiraladığımız araba şurada", then using "sizin" in the next sentence is sort of necessary as you are comparing. So you cannot say "better" without the context
İşaret zamirleri. Demonstrative pronouns.
Şu this(one), that(one)
Şunlar These, those
The plural form of the demonstrative pronouns take the plural suffix '-lar' and the letter '-n' is added before the suffix '-lar'.