Which Macbook should I buy?

Hi everyone,

(I'm not from the US, so all prices are in €, but I think that makes no difference)

I want to buy a Macbook Pro Late 2013 with Retina Display.

My needs are:

1 Running 2-3 virtual machines at the same time (one is Windows 8, rest Linux')

2 Having Xcode on OS X opened, and Visual Studio on Windows

3 Doing some photo-editing (not too heavy work)

4 Once in a while light gaming (not that important)

5 Travelling a lot

6 Every now and then some (heavy) Mathematica-Calculations

7 Having MANY tabs opened in browser

8 256gb SSD should be fine I think

At first I was sure: I'm gonna go with the middle-13" Model and upgrade the RAM to 16gb (1699 €). 16gb for two reasons:

1) Might need it for my needs (???).

2) Future proof (also better to sell [?])

But I could also go with the high-end 13" and therefore have 512gb SSD and 2.6ghz processor, with only 8gb of RAM.

This would cost (1799 €) the same as [13" 2.6ghz 256 SSD AND 16gb RAM].

But when I'm paying so much anyway I could also go with this configuration which I can buy for 1849 € at a local reseller:

[_15"_ 256gb SSD 8gb RAM 2.0 ghz quad-core]

I have heard, that the 15" quadcore is up to 90% faster then the 13" dualcore.

My questions in summary:

1) How important are 16gb RAM, considering my needs AND the future-proofness (might need it in 2 years, could maybe sell it relatively better compared to 8gb)

2) How much faster is the i5 2.4 ghz compared to the i5 2.6 ghz?

3) If I go with the 13" 2.4 ghz and 16gb Ram: Wouldn't it be better to take the 2.6 ghz (even if there is NOT a considerable surge in performance)? Because at the time I sell it, people might say "Ugh? 16gb of RAM but only this 'weak' processor?"

4) How portable is the 15" model? I know, it's way faster than the 13", but if the 13" can handle all my tasks AND is better for travelling, why not save 150 € (50 € respectively)?

THANK YOU (in advance)!