I don't know what your pricing is like, so I'll just use US prices. Was just looking on Apple's site and I don't see the option to get a 256gb SSD on the i7 model, the 512 is the minimum. That surprised me, can you actually get the 256 in Spain?
Anyway, in the US it costs $600 to upgrade to a 2tb internal SSD. I see the 2tb Samsung X5 external thunderbolt SSD on Amazon for $550. To me it would be a no-brainer to go with the 2tb internal SSD on the Mini. As you can see from the screenshot I posted, it still out-performs the write speed on an external thunderbolt SSD and it just isn't as "kludgy". Aside from that, the internal SSD can never be upgraded so it makes a lot of sense to get the biggest possible.
That is what I did with my own Mini and I have absolutely no regrets. Even if it was (for example) $200 more for the internal 2tb SSD, would do the same thing. I don't think you would ever regret having a bigger internal drive. Now, I do understand that saving a couple hundred dollars might make a big difference for some people, but I would buy the biggest internal disk you can possibly afford. If you are keeping the Mini for a number of years, it really isn't that much.
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