I have my eyes on a Waterman Hemisphere, black, with gold trim. However, I am not sure what nib size to go for (pen will be purchased online). Until now, my only fountain pen is a Parker 45 with a medium nib, which works well for me. Elsewhere on this board, I have read, that the Hemisphere medium nib is a bit on the broad size. Should I go for a fine nib instead, or will the medium nib do just fine?

I don't have much to compare it to, but I've been using a Waterman Hemisphere with a M nib for many years and it seems excellent. It's a bit finer than my new Conklin All American M. Great pen if you ask me. Looks nice too!
Wad.

I'm a B lover, so I can't really help you on this one. I just know a B Visconti and a B Waterman seem pretty equal to me.

I own both pens, I do find the parker 45 medium nib to be slightly finer than the hemisphere, I use the hemisphere as my everyday pen and find it a joy to use, very smooth, good weight. I own two hemispheres and find that they do not perform very well with waterman ink, I have used parker, sheaffer and caran d'ache ink and they all perform very well in the hemispheres.

What problems are you seeing in the Waterman ink with your Hemispheres?

The are slow to start and the ink flow constantly cuts out, this only happens with waterman ink,