A 4x6 joist is only slightly stiffer than a 2x8 joist. But at an 8-foot span, 2x6 is plenty stiff (Code max deflection occurs around 10-foot spans). So your question really relates to joist spacing, not joist span/size.

I think 32" O/C between joists is stretching it. It will certainly work, but I think it might be a little bouncy in spots for some people -- comparable to, say, using 2x6 joists 16" O/C with a 12-foot span. I myself would probably stay at 24" O/C. In your case, it's easy enough to simulate a Best Case with some cinderblock and a couple of planks.

Keep in mind that if you ever decide in the future to install composite decking surface the joist span required is less then that of 2X6 Redwood. Very few composites allow for 24" O/C joist spacing.

I would do the 24" O/C, but I'm looking at not using a ledger but hanging the joists straight off the house with simpson hangers at the house studs below the sill plate. The area does not see a lot of rain, but I figure that this will minimize the contact with the plywood/T-111 siding of the house and would be even better than a ledger with flashing.

If I go 16" O/C, I'll probaly go to 2x6 joists. At about 10" off the ground, am I being too concerned about airflow? Should I install some vents?

At only 10 inches off the ground, why not consider a stone type patio?

I like stone patios but I don't think it would fit the rest of this house, which is in the hills in a wooded area, with three other redwood decks already attached to the house (all one story or more off the ground unlike this one which will be at ground level).

What about a patio with a redwood pergola? Can you post a few pics?