Regardless of if you go MAC or PC I think the 1TB Fusion drive is a huge mistake, you should get the Crucial M500 960GB SSD. M500 is 10x faster, the Fusion drive will bottleneck your system.The 960GB SSD would be the quickest option, but you're screwed when you pass 960GB in total space used.
The 256GB + USB 3.0 externals would give you the ability to keep your libraries on the fast internal SSD, and keep your ever ballooning photo library on far cheaper external drives. You'd still see a major performance increase.
That said, down the road, if you ever wanted to, or SSD costs decreased enough, you could buy and put that 960GB SSD into an external and get the same benefits as having it internal. Thunderbolt could easily handle an SSD with up to ~900MB/s read/write speeds.
Will probably get the x-rite i1 Display Pro soon, the cheapest I have found is £159 on Amazon.
Could someone tell me if there is any difference between the dell u2413 and u2713h? Is it just the screen size which is different?
The resolution difference between the 2413 and 2713 is substantial.
1920x1200 (24) vs 2560x1440 (27)
Aspect ratios are slightly different at 16:10 (24), and 16:9 (27).
Other specs are almost identical.