In terms of ink usage to create a given image, you won't see a particular difference. The gains to be made are through using larger ink cartridges which are sold at prices that are less per ml than the smaller cartridges.
There may be some queries or concerns about cleaning cycles and usage of ink compared between the models, but for the vast majority of users this is not an issue (for those for whom it is an issue, I am not diminishing in any way the problems that you may be facing - let's not turn this into something beyond what the OP has asked). Again, even with cleaning, the larger carts provide a saving, but they're no more or less "efficient".
My advice would be to find a dealer which has a 4900 and a 7900 (or 7890) in stock and have a look and try them out. I know you didn't mention a 17" but have a look. Between a 24" and 44" other than size, there's no difference (so if they have a 9900 or 9890 in stock, look at that instead).