Stay away from Oakey (between Dalby and Toowoomba).
If you go Dalby choose a place on stumps or a hill, when it floods it normally will go under more than once.
Personally I would choose Toowoomba over Dalby.
Other posters have made the comments that at least Toowoomba's economy is diversified and can withstand some dowturn in mining.
To hold a place in Dalby would be leaving yourself completely leveraged to the mining industry, not sure if you have seen the news recently but it's been a blood bath in coal mining. Approx 11,000 jobs have been cut from the coal sector in the last year.
This is a trend that certainly isn't about to change either.
Coal miners have been crunched between high costs, high Aussie dollar, increased supply on market, reduced demand and rising anti coal mining sentiment in the land of Oz.
The only thing propping up mining in QLD is CSG. Not sure how many of the job losses in coal are been absorbed by gas industry though.
I personally think that a lot of the inflated property prices in towns like Emerald, Dalby, Chinchilla will be hit once reality sets in regarding the future of coal.
Lower rents, higher vacancy rates, no doubt people will want to sell putting downward pressure on prices.
So IMO from a growth perspective Toowoomba is a much better option.