bit of advice needed for first mill

[m_c]

There's only two chuck versions, R8, and MT3.
The imperial/metric refers to the leadscrews/dials.

R8 has the benefit it should be self releasing once you slacken the drawbar, whereas MT needs a hit with a hammer to release it. R8 is traditionally more an American thing (Bridgeports used it), whereas MT is more a British thing (although british milling machines usually used a proper toolholding taper in the form of a INT/BT taper, with MT being used on drills/smaller milling machines).

For hobby use, there's not really any major difference between them, and if you've not already got any tooling, I can't think of any reason to specifically use one or the other on a machine that size.
If it was a larger manual machine, I'd say MT as it gives the option of using MT drills (more common for anything above 1/2" or 13mm), however a X2 will struggle with drills above that size in steel, so you may prefer the convience of self releasing tools.