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I think 15" is unwieldly on planes and also on your lap. You need to do a lot more balancing. I think the screen size difference is minimal.
 
I've only had one 15" model in the past. This will be my 2nd 15" model. I'm just curious to find out how much better the new 6 core i7s are compared to what I had before.

Define "better." They both run the same software, right? And rather quickly, I might add. Fast and faster, that's about it.
 
I prefer the 15". As I'm getting older my eyes seem to prefer a much larger screen. Also prefer the extra power and dedicated graphics card when I need it.

Carrying it around hasn't been a problem. A good backpack will make that a non issue. But I've carried my late 2013 15" MBP in messenger bags with no issue. Though getting older, my shoulders prefer a backpack now a days.

On planes and tight spaces I usually just use my iPad. But when I needed to get work done, I've sat in economy seats with my MBP. It was tight, and a bit uncomfortable but it just made me swear I get work done before the flight :)
 
A comparison between 13” and 15”


This has been what i've been more curious about. the performance difference this time around between the 13" and 15" is less than it has been in the past since the 13s are now on quad cores
 
I prefer the 15". As I'm getting older my eyes seem to prefer a much larger screen. Also prefer the extra power and dedicated graphics card when I need it.

Carrying it around hasn't been a problem. A good backpack will make that a non issue. But I've carried my late 2013 15" MBP in messenger bags with no issue. Though getting older, my shoulders prefer a backpack now a days.

On planes and tight spaces I usually just use my iPad. But when I needed to get work done, I've sat in economy seats with my MBP. It was tight, and a bit uncomfortable but it just made me swear I get work done before the flight :)

This. Totally. Older eyes. The spec bump is icing on the cake.

But each individual's needs are different, trading screen and power for portability.
 
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