Do these two DPDT relays function the same?
The enclosed one is definately better for the motors. Check the spec table below the description:

 Open relay:  HP @ 240V -&gt; 1-1/2 
 Closed relay:  HP @ 240V -&gt; 3 


This means that the contacts are more rugged against the inductive feedback of switching a motor on and off, and therefore will last longer. The open relay, however would be better for the ammeter as it appears to be connected not to the motors, but only the boiler and would benefit from the extra resistive load capabilities, although the closed relay would do just as well, since it is above spec. Basically, use the enclosed relay for motor control, and the open one for resistive load if you are not concerned about spending almost $20 extra.