What would load quicker theoretically:
2 loads with a where clause on PORT_NO for XXX values showing field MKT_VALUE
1 load with an If(PORT_NO = XXX , MKT_VALUE)
I am not concerned with memory, as it is relatively small table loaded.
That depends entirely on what you plan to do with the values that don't meet the PORT_NO = XXX criteria. If you plan to ignore them, I'd use where. If you're going to keep them, but do something special with the matches, use if.
Retrieving data ...