Which float switch to use for my A/C unit?
You want a switch rated for AC at at least the mains voltage you are using and able to handle more VA or Watts than your load. 

Temporary paragraph:
 Please say what is being switched.
 Is it just the 40 VA fan load or heating as well? | 



The 48 VAC switch is almost certainly unsuitable - see below. 

The switch you linked to (photo below) is rated at up to 230 VAC and at 11 Amps current.  It will work with AC voltages lower than 230 VAC as well.
 At 110 VAC a 11 A switch is rated at 11 x 110 =  1210 VA - say 1200 VA or 1200 Watts.
 At 230 VAC a 11 A switch is rated at 1about 2500 VA or Watts. 

If you are in Texas then your mains may be 110 VAC unless you are operating betwen two transformer halves - but voltage wise it should be OK regardless.
 The 11A raing may be too low or very ample depending what you are switching.

The electrical rating of the Payne PF4PMA that you refer to is shown as 40 VA. ie your 11A switch will handle it with ease.
 BUT there is also reference to a 3 to 30 kW heater. Your switch would die just looking at that load.
 (2500 VA is ALMOST 3000 Watts BUT you really want some safety factor. 30,000 Watts is "unusual" in a home installation). 



IF the unlinked-to switch is switching a lower voltage load at the output of a transformer then it MAY be suitable. Voltage would need to be &lt;= 48VAC and current &lt;= 5A.