Topband: best core material?

[James Rodenkirch]

I have a schematic for a delta shaped loop that shows I'll need an 18:1 transformer to transform the 950 ohms of the antenna to 50 ohms (feeding it with 50 ohm coax).
 
One transformer diagram shows an FT-140-43 core being used.
 
BUT, looking over some of Tom's, W8JI, write-ups, I see where he uses 73 material instead.
 
I see where 77 material replaced 73 material so -- is an FT-140-77 the mo betta way to go?  
 
 
Thanks, in advance, for any advice/info.  Jim Rodenkirch K9JWV