Comparing the syntax of a 6500 vs. 3845 will mostly depend on whether the 6500 is running a very old CatOS and/or hybrid OS or the newer (and likely) native IOS. The latter should be very much like the 3845's. Of course, there will be differences based on IOS version, feature set, installed hardware, and support for L2 features (although if your 3845 has a switch module, that too will be somewhat similar) on the 6500. Generally, though, if you're familiar with the 3845, likely the 6500 won't be too confusing.
