I say do the swap. No idea what an ST1 fetches at present, but if it is a straight swap or thereabouts I'd say the 321 is much nicer so go for it. Don't get me wrong the unique panda dial, radial numbers and brush finish make the ST1 streets ahead of most (all?) Speedy Pro case LEs but for me the straight lug is just a nicer thing to wear.
I may be swimming against the tide here, but as a former owner of the ST1 I hope my opinion has perhaps some worth. Of course maybe the best compromise is to keep the ST1 and buy a FOIS if you can stretch to it - it's a helluva lot cheaper and has a leather strap too, though maybe not as nice as the ST1 catch stitch.