I have experience with XNA and Unity3D. I have also built my own engine with Tao and C#. I am in last year of college and I want to make a small RTS game demo, nothing fancy I won't bother much with the art part. I will leave objects untextured, just drop some light.
It is a big decision, which one to choose XNA or Unity3D or maybe else? I definitely won't build my own engine since I will waste lot of time that way. I am looking for an engine/framework that will allow me to concentrate on higher level stuff like AI algorithms rather than do my own texture loading and stuff.
More details: I will try to copy stuff from Starcraft2. My models and environment won't be textured though. I won't bother with sounds either, just some simple explosion and shooting sounds. The player will play against AI. I will go for the Starcraft2 mechanics/logic, not the skin. By skin I mean art stuff. At the end I would make small videos of gameplay showing the AI decision making process by printing text, so that a potential employee can see what is actually going on.