the two are interchangeable, as some owners put mk2 bumpers onto a mk1 fr a diff look. the foggies are very diff on the mk 2, squarer, sharper corners, flush fitting to the surface.
but mainly it's down to the shape, the mk1 is a bit bluff and upright, compared to the sleeker mk2. plus the 2 does look good all scrubbed up, with a red stripe along it!
as alternative, there is a far left way to go, as found by a member a very long time ago.
find one of these:
the bumper is almost a direct fit to the uno......
finding original mk1 uno turbo bumpers is hard as hell, i was lucky when tippex needed a black bumper to replace the painted plastic, i had a full set, lying around. last time i saw a bumper listed by a supplier who actually had one in stock, it was three figures to buy.