Hello and welcome, we're a friendly but small bunch so lots of input needed when you have committed
I've owned both the 312 and 351 and found both 'nice'. Of the two my clear preference was the 351 as it has so much going for it. The Ti case is easier every day than the heavier 312. The brown dial and gold hands are
and i think nicer. The 320 which you've seen is of course a 312 with GMT, though the 320 has a couple of other nice features, a 'seconds reset' when you pull the crown and a PR indicator on the back. The 312/351 do not have those features.
If your choice is a two horse race of 312 or 351, i'd pay the extra for a 351 every time.
HTH