Yes, the s900 has 6 ink tanks, the i850 has 4.
I own the s900 and it prints photos extremely well.
However the i850 is a newer model and is said to print comparably, if not better than the s900 and at about half the price.
i850 has a new print head with smaller ink droplets.
Debateable how much better 6 vs 4 ink wells are, theoretically 6 might save a slightly on ink costs over time.