Good question .
Well what SBST and SMRT are looking for a few factors.
The most important is the cost of the bus.
The maintainance cost, the purchase cost and the cost of the spare parts.
These are the following factors they are looking when evaluating .
1. Chassis , low floor body and configuration must suit SBST or SMRt's needs.
2. Engine : The engine must be fuel effiency , reliable and enviroment friendly. They are probably looking at Euro 4 engines.
3. When it comes to bus types, they will more or less stick to the same brand for example like SBST = Volvo and SMRT = Mercedes because they are already operating the brand and makes maintainance easier. Unlikely they will get a new type . And some more they get discounts from the companies they purchase most of their buses so this is another plus point why SBST had been buying mainly Volvo buses .
4. Tyres : Probably stick to the same company.
5. During evaluating the new bus , the following factors must be considered:
Is the bus suitable for singapore roads ? Is the engine able to run more then 12 hours per day ? Is the engine powerful enough to climb up the steep slope with a full load of passengers ? All these are considered during evaluation and there are trials on the new buses before the companines make a decision.
SBST's choices = Volvo , Mercedes, Scania or Dennis (I doubt)
SMRT's choices = Mercedes , Scania , Dennis or DAF
You can't see SBST purchasing DAF buses or SMRT buying Volvo buses.
There are some areas I have missed out, I hope some bus experts can carry on where I left out.

Cheers !
Hope this helps.