I finished the vampire side and now playing on dawnguard. What actual difference between becoming the vampire for a while and later curying it to become the werewolf again and sealing part of your soul into gem? Will I be able to return if I cure vampirism? And Serana mentioned something about becoming weaker. Do you loose were perks?

I went in as a wolfie Dawnguard and chose to have a part of my soul taken.
- While in the Soul Cairn, my character had (if I remember correctly) -15 to magicka/health/stamina.
- That portion is considered payment for being able to enter the realm, and only applies while in the Soul Cairn.
- Upon exiting my stats were back to normal.
- I was able to re-enter wthout incident.
- If you wish, you can recover your soul fragment before exiting the Cairn.

That's as much as I know so far. Meaning I have no idea if you can re-enter after retrieving your fragment.
Maybe another member can comment on actual experience with that aspect/possibility.
Hope that helps

What level is your char? I ask because mine is 36 and she lost 45H/S/M. Perhaps the handicap is level related.

mmm,yeah tryed the sealing part. lvl 46 lost about 80points to everything

Update:
- I recovered my soul fragment, quested around Skyrim for 2 months or so, then returned to the Soul Cairn.
- No issues entering or exiting with werewolf blood and my stats remained whole, even while in the Cairn.
- Makes one think the hit to stats is only for the MQ portion.

I lost that same amount and my character was only level 24 at the time.