Effect of the processor on standard use cases?

Ive been reading a lot, and decided on the basics for the new computer, but I have a few questions. First, some background:
I got a new job that has me traveling a bit, but normally i leave my computer at home.
I do not do anything crazy with it. Maybe play a few games, but that is far from being a primary use case.
I am leaning towards 256gb hard drive and 8gb ram because:
a) I have a 256gb external where i store pictures and movies
b) as I said, I dont do anything crazy enough to warrant 16 gb of ram (or convince me otherwise?)

First, and most important
What processor should I use?
What is the real difference between i5 and i7?
Between 2.0, 2.3, 2,4, 2.6, 2.8 ghz?

Also a question- what size is good?
I am leaning towards 15 in because:
a) I leave it at home a lot
b) 15in isn't that bad for when i need to travel, as I have grown to understand.
c) Watching movies, reading, writing, doing presentations on 15in is better and easier on the eyes than on the 13in

The only reason that I see me getting the 13 in is if the size is really that much better, lighter, and does not make a big difference on how i read, write, or watch movies, or play the occasional game.

Money is not the main issue, im more concerned with getting the system that is perfect for me.

I very much appreciate all of your advice in advance!

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Or am I overthinking this, and I should just get a MBA top of the line?

 

Is the 15 airplane friendly? I leave the computer at home when I am on location, but I fly every now and then, (10hrs+ per flight).

And, in general, does i5 v i7 matter in terms of performance or battery? I remember reading that i7 has better battery, then i read that it is worse. What do you find?

Also, theKitch, i see you have both a 13 and a 15 in. Can you share your experiences with them?