Ken,
One other factor to consider is your customer base (if applicable.) If printing only your own work it is very fair to say while the differences in some cases are subtle and a bit more visible in other files, they both would yield a great quality, sellable print. A good point was also made earlier in terms of the canvas itself being a factor in overall print quality.
If you are printing for artists you perhaps should consider the 9900. It does have advantages (even if only slight ones) and we are all more educated these days about technology and overall printer capabilities. Simply put, the educated customer could reasonably choose your competition if it means they have a 9900 and you have a 9880, even if the difference on canvas is slight, the perception could very well be that his prints will be better.
In the end, you know your business best and either way you will have a print that you can sell any day of the week but perhaps there are some less tangible differences that also could help make this decision.
Regards,
Gary