Vindolanda is indeed a wonderful site to visit, but you can't go half way round there world, visit Vindolanda, but not go to the most famous (and extant) part of the Wall itself.

Vindolanda is great because of all the reconstructions, Housteads is great because it's all original.

Ideally do both.

If you can only do one then Housteads is for those who already know what things looked like and can stand in a pile of rocks and visualise the original site, Vindolanda is for those who want to learn how the originals looked.