I just ran a simple test;

LOCAL lnStart, lnEnd
ldDate = DATE()
lnStart = SECONDS()
FOR x = 1 to 100000
	ldLastDay = GOMONTH( ldDate,1 )
	ldLastDay = ldLastDay - DAY( ldLastDay )
ENDFOR
lnEnd = SECONDS()
? "2 lines: ", lnEnd - lnStart

lnStart = SECONDS()
FOR x = 1 to 100000
	ldLastDay = GOMONTH( ldDate,1 ) - DAY( GOMONTH(ldDate,1))
ENDFOR
lnEnd = SECONDS()
? "1 line: ", lnEnd - lnStart

And got the 1 line method to be slightly faster than the 2 line method. The actual times varied on each run but an average of 0.591 seconds for the 2 line version and 0.530 for the one line approach.