That's not true at all, there is a very large sales difference between FF and DQ, because FF is in a huge sales dive. If you compare the last FF, FFXII at 2.42 million vs. DQVIII at 3.61 million, you are looking at 43% difference in sales.
As for how being on a handheld will make a difference, it's the DS. The DS is a phenomenon in Japan, with core games and casual games alike selling like crazy on it. New Super Mario Bros. sold 4.75 million on the DS in Japan, Pokemon Diamond and Pearl sold 5.29 million in Japan. Do you really see one of the most popular franchises in Japan underselling it's predecessor now that it's on a much more popular game system?
FF12 wasn't turn-based and FF13 will be. It will also be the first on the console. Now, do I expect DQ9 to outsell it? Yes, of course. But FF13 will be much more of a system seller.
Finally, if you think there is a huge difference in sales I'll just disagree and we'll call it done. I think the numbers speak for themselves. DQ8 didn't sell as well as DQ7 or DQ3. Does that mean it's going to continue to decline?