“Better” depends on what you mean. These two cameras sit in different parts of Nikon’s lineup: the D300 is a higher-end, more professional body, while the D3100 is an entry-level model.

From the answers given, the D300 is generally the stronger camera overall. It was designed as a more advanced body, so in most important handling and performance areas it’s considered better than the D3100.

The D3100 may have a small advantage in newer-sensor areas such as resolution and high-ISO performance, but that doesn’t make it the more capable camera overall.

So the simplest summary is:

• D300: better overall, more advanced/pro-level body
• D3100: simpler entry-level body, with possible slight edge in resolution/high ISO

If you’re learning, that doesn’t mean your D3100 is “bad” — it can still be an excellent camera to start with. The best camera is the one that fits your needs and that you know how to use well.