What are the differences between SQL I/O Sim and SQL IO tools?
SQLIO is a generic tool for testing your disk subsystem. It allows you to specify whether to test random/sequential, data block sizes, queue depth, threading, etc.

SQLIOSim is a tool that tries to emulate the patterns that SQL Server would expose to your system.

Usually I'll use SQLIO to test the subsystem when benchmarking for raw specs. Once satisfied with my SQLIO results I'll run SQLIOSim to get a real world run through of the disk subsystem.

You might want to see Brent Ozars tutorial on SQLIO, which also mentions SQLIOSim:
http://www.brentozar.com/archive/2008/09/finding-your-san-bottlenecks-with-sqlio/