Wheelchair Ramps

For heavy use, harder woods are preferable, and redwood and cedar are relatively soft. They'd work, but they'd tend to splinter with heavy use. PT wood is southern yellow pine which is a fairly hard wood, but the pressure treatment makes the wood prone to checking (looks like cracks, but they're not structurally significant) and it shrinks a fair bit. It's still the most common wood for ramp decking. Ipe (pronounced eepay), and known by other names, is a very hard wood that looks good and will outlast all of the others.

If your ramps won't experience a lot of traffic, redwood is harder than cedar, and since you're in California I'd imagine it would cost less than it does here in NY.

Rico's recommendation of IPE for wood decking is a good one. The wood/plastic composites like Trex and CorrectDeck have excellent slip resistance. These require less maintenance than wood, but are more expensive.