We are a team of 15 looking for something which is easy to use and track work loads.
Pivotal Tracker - More technical features specifically for software development, steeper learning curve, good bug tracking.
Slack - Good for office-wide communication. Easy to use for everbody.
Bugherd - Bug tracking system which integrates into the user-facing components of your web-app. Makes gathering feedback/bug-reports from the public very easy.
If it's a case of the latter then I would advise to look into adopting a methodology and a systemized way of working before locking yourself into a tool.
In my experience there is a big correlation between using a methodology and having a solid foundation to build a solid business.
Slack - Quick communication and easy integration to other apps/tools
Don't like: Jira, Whiteboard (like.. literally a whiteboard)
Want to try: Zenhub
TBH, though, I struggle to find any other meaningful negative differences.
My biggest peeve is that management always want to configure it to do time-tracking. This makes engineers hate the tool and lie to it, making it useless for bug tracking.