Dream Job vs Skill Level 9
Solution 1:
If you have the option, I say dream jobs.
Why? Dream job bitizens provide better values for you money. If you fill a floor with 9's, bbut no dream jobs, the effect you get is a 27% discount on restocking (it's always a total of the staffs' skill of that floor type, so 9*3).
That might seem like a good thing on it's own, but if you have staff doing their dream job, you get twice the stock each time your restock and you get discounts on restocking by their skill level (they probably have at least 2-3 skill points, probably even more). It can be compared to a 50%+ discount.
(All my calculation assume you meant all 3 staff have their dream jobs, of course, the real percentage varies if it's 1 or 2, relative to each floor items' stocking cost vs stock count)
Solution 2:
Do both where possible. If someone doesn't have a 9 in their dream job I evict* them from my tower. If their dream job is one I don't have yet, they can stick around, but they shuffle through jobs wherever I need them. If some has a 9 in their dream job, and it's one that I have the right store for then they get to work forever.
*Of course, if someone has less than a 9 and their dream job is one I have, I'll hire them first for the 2 bux, then evict them. I am a heartless and efficient landlord!
Solution 3:
So it appears as though giving a bitizen their "dream job" will double the stock without increasing the stocking price.
- Dream Job employee will double the 1 Coin value stock
- Dream Job employees will double both the 1 Coin and 2 Coin value stocks
- Dream Job employees will double all stocks
As for the "skill level", here's what the wiki has to say.
Each Bitizen has a certain proficiency with each business category (Food, Service, Recreation, Retail, Creative) ranging from 0 to 9. Based on the skills of the employed Bitizen, a discount will be applied to the cost of restocking the items or services for that business. The discount is equal to the sum of each employee's skill level. The maximum discount is therefore 27%, the profits of which can certainly add up for high cost floors.
So basically high skill bitizens cause the cost of stocking to go down. Dream job bitizens double the stock quantity for free.
The ultimate combination is obviously all three staff members will have their dream job with a skill level of 9, but until those stars align, you would need to decide on which approach to take in your situation... Probably "Dream Job" because you're essentially getting your stock at a 50% discount instead of a 27% discount.