The Mont Blanc stays in my shirt pocket during the day at work, is locked in my desk overnight. The Cross Nile stays in my messenger bag as you always need a ballpoint for filling out carbon copy forms, etc.
I don't do fountain pens as I'm a lefty, and the Mont Blanc rollerball cartridges are AWFUL. So, I just modify Cross gel cartridges to make them fit in the Blanc and it writes like a dream.
At home I have a Faber Castell Tec Flex and Visconti Van Gogh Self Portrait...both ballpoint shells that I use Parker gel cartridges in.
One of these days I will buy myself a Mont Blanc ballpoint in burgundy and stick a Parker gel in that as well.