My company is in this exact situation and we've chosen BitBucket because the pricing model per contributor is a much better fit than GitHub's model for our usage pattern.
Is that not your experience?
We currently have 9 people accessing our 12 source repositories (9 BitBucket users or 9 GitHub collaborators). With BitBucket we pay 10USD/month, with GitHub we would have to pay either 22USD/month (the Medium plan) or 25USD/month (the Bronze plan).
In the end, BitBucket's pricing structure is much simpler for us and fits our model much better.
Github is no doubt useful, but don't ignore the others -- this is not facebook.
One thing I love about Google Code is that they allow multiple repositories per "project". This is very important for when you want to share the same issue tracker, wiki, downloads etc because the items are related. Many of the overall projects I work on have a server piece, some tools and an Android client. They all work together as a whole. When there is an issue, it is often not apparent where the blame actually lies and which component will need to be changed to fix it. (Eg a font being too large in the Android client could have been generated from the server which served up templates made by a tool.) Google rule out private hosting so they can't be used in this scenario - http://code.google.com/p/support/issues/detail?id=1829
As a regular user of Google Code, Bitbucket and Github, the arbitrary differences are very annoying. Each uses a different wiki syntax, and most relevant each has a different syntax for closing tickets in a commit comment - they are similar but not exactly the same.
Right now just limited to "High", "Medium" and "Low", but we could have whatever level of granularity we want. We can also color code the tags (red = high, orange = medium, yellow = low).
The thing I really miss on github is a place to stick a numeric estimate for how long something will take to help with knowing how much capacity we have left in a release/iteration.
trac does a really good job on this side of things, especially a nice place to provide (and sort) milestones, show their progress, a little bit of writeup etc. This is an example:
Github brings us no benefit then.