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I'm going to use it for work with game engines and Blender (modelling tool). Photoshop (not large files) and sometimes Premiere Pro.

This laptop won't be my main working machine, but I still want to be able to do some "serious work" even if I'm not at home. It seems that 13" is quite good for this too (rendering time: 13" = 35min, 15" = 23min)

8GB RAM is enough for me, but I want to get 16GB cause I'm buying this laptop for 3+ years

I love 13's compactness, but 15's screen size and power is also nice. So...

oh yeah, the 15" all the way! And 16 gig with the NVIDIA 750m is a good idea.
 
I'm going to use it for work with game engines and Blender (modelling tool). Photoshop (not large files) and sometimes Premiere Pro.

This laptop won't be my main working machine, but I still want to be able to do some "serious work" even if I'm not at home. It seems that 13" is quite good for this too (rendering time: 13" = 35min, 15" = 23min)

8GB RAM is enough for me, but I want to get 16GB cause I'm buying this laptop for 3+ years

I love 13's compactness, but 15's screen size and power is also nice. So...

Definitely 15" for your usage needs.
 
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