For me: it depends!
Blyxa grows so quickly and branches so easily that it fills space nicely. It can outgrow its space and crowd neighbours though.
E. Vietnam behaves itself so well it can be a true foreground/front of midground plant. It holds its own against aggressive neighbours too, so can border any plant.
Blyxa colours up nicely, although I do love the rich green of Vietnam.
TLDR: both are great plants! Which one depends on where in the tank you’re planting it, how much height and width you need and whether you want a splash of colour or a green block!