Help needed with choosing the right ThinkPad

Hey out there!
I am currently planning to buy a refurbished ThinkPad.

I found offers at similar prices for a T450s and a X270.
The T450 has an i7-5600U CPU and 8 Gb RAM, the X270 has an i5-7300U CPU and 16 GB RAM, both have full HD IPS-Panels.

Apart from the display size and weight, I am uncertain as to which of these machines will fit me better.
Coming to my questions:
- Does anybody have experiences with running Linux on these machines? Any known problems? I am planning to install Linux Mint Debian Edition or some other Debian derived distro.

- Are there known issues when upgrading the RAM on the T450s? The 8 GB in the offered machine will not always be enough, but the T450s is a bit cheaper which would outweigh the cost of extra RAM.

- Are there noticeable differences performance-wise? The i5-7300 is two generations newer, but "only" an i5, and has 3 Mb cache instead of 4. Apart from office work, for which both machines should be way enough, I will also do some photo editing and to a limited extend video editing, obviously with an external monitor.

- Does it make much difference that the T450s has DDR3-RAM as opposed to the DDR4 in the X270?
- One plus of the X270 could be its USB-C-Connector. But apart from charging, I read in a review that there are issues with using it as graphics output for high resolution monitors. Any experience with that?

- Generally, are there reasons that I have not mentioned that would make it smarter to choose either the X270 or the T450s, apart from size an weight?

I am still a happy user of a T61 which runs smoothly with Linux an 8Gb of RAM, but now I need a second machine which is a bit more mobile, faster and supports Full HD. I'd rather always have a ThinkPad that's some years old than buying a newer "consumer" model at a similar price, because I want a reliable machine that really can take being carried around a lot.

I'm happy for any help with making my choice!
Thank you in advance!

All the best,
Tom

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