I would go for a commonly used framework. As you are interested in path planning, pick a framework that already has simulated robots, and just write the planning services you need to generate paths for it.
ROS is very cool, and has quite a large selection of useful perception services, as well as the full PR2 stack. It uses gazebo (part of player/stage) for simulation, and includes a few off the shelf robots (erratic would be ideal for this). Its quite new, but very active.
MRDS has several suitable robots, and frankly better simulated environments then ROS, but not quite as much by way of high level services. The lego robot simulator would be fine for what you want to do, and gives you the choice of validating on real hardware cheaply in the future.
These two have the lions share of developers at the moment. Really the decision boils down to whether you are happier developing in visual studio under windows, or on linux platforms. Both are free for students (MSRS is under ELMS / dreamspark programs), both have a learning curve, both have pretty good community support. For MRDS, go sign up to the forum, for ROS, join the mailing list.
