I've just arrived at Serana's family castle with her and we've located the portal, but I either have to become a vampire or have my soul partially trapped to enter. Which do I choose? Can I get my soul back after returning? Or is there a cure for the vampirism?

Yeah, there is a minor quest at the end before you get out where you go looking for your soul...pretty easy.

Yes, you get your soul back once you leave the Soul Cairn. You'll lose 45 points of health, stamina and magicka while you're soul trapped, but I don't have any real problems in there anyway while doing this quest while soul trapped. That's what I pick.

If you choose Vampire Lord form, you can cure yourself later in the normal way you cure vampirism.

Vampirism

Along with what everyone's said, if you pick vampirism it'll be difficult to return to the Soul Cairn if you cure yourself. You'll have to get infected again to go inside for whatever reason.

If you get soul trapped, you can go in and out all you want once you complete the quest and get your soul back.