Music has much more weight definition on the WS55 and bass is much tighter on the WS55. The isolation while not the best on the WS55 you actually will get isolation with music playing you can't hear anything outside at all. The WS55 does have a bit warmer sound presentation and is musical the Portapros are more airy as they are open. Mids are kinda muffled a bit on the portapros. The mids on the WS55 are cleaner and smoother at the same time. Highs come in cleaner on the WS55s as well.
The portapros have been a good cheap go to portable for a reason. For the bux they produce a good sound but they are out classed by the WS55. The sound signature while a bit different instruments have better separation and definition. The bass presentation is similar but the WS55 has the better quality bass here. Sub bass is much weaker on the porta pro.I noticed since the mids on the porta pro is a bit recessed the high notes seem to be more prominent on them giving them the V shape sound so the WS55s are balanced better as well.
Now these do have some big bass so if you want even a more balanced closer to a neutral sounding portable I suggest for the same amount or less you should look up these.
ATh-ES55. These are not bassy headphones but are more balanced while providing a similar amount of isolation. I think these will be even better for you vs the WS55s. Especially since you don't want a bassier headphone.