Should I see this film in 3D or 2D?

I'm seeing Harry Potter 7 Part 2 next week in 3D but I'm having second
thoughts about it now and wonder if 2D will be better.

I wear glasses generally (short-sighted) and was planning to keep them
on during the film and simply place the 3D glasses over the top (if
they are big enough)...Will that affect the quality of the 3D images
and/or give me a headache? I thought it might ruin the effects as my
glasses don't cover my whole eyesight - they only work for straight
ahead...so I can't just roll my eyes upwards to look up, I have to
turn my whole head upwards (hope that makes sense!)

And will I get a headache watching the 3D film without the 3D glasses,
say if i decide to take them off? I heard it can be quite blurry to
watch a film without the glasses but some people have also said that
they've been able to continue watching a 3D film in perfect clarity
without them.

I'm not really familiar with the different types of 3D but the cinema
says it uses Digital 3D/RealD 3D...which means nothing to me...Is that
high quality?

Quite a few questions, I know, but I'd appreciate it if you could
answer them all! X