Had the exact same dilemma. Came very close to a black faced ExpII, and still feel it's a beautiful watch.
For me however, in the final analysis, I opted for the GMTII because:
It's my one and only Rolex, and so I liked the "classic sports Rolex" look with the rotating bezel etc.
It's more versatile with the ability to change the bezel to different colours.
I felt that the ExpII looked a little small as the pale coloured s/s bezel "shrinks" the watch. Having said that, I love the "machined" quality to the brushed stainless against the black dial.
Here's mine. It came with a Pepsi bezel fitted and its original Coke bezel separately. I switched it back to the Coke because, whilst the Pepsi is iconic, for me it was just a touch "Fisher Price" with it's brighter colours, I prefer the more muted (but still adding a dash of colour) Coke.
This is how it looks now:
And this is how it looked before: