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.