Just finished building my SPR-ish upper w/ a WOA 18" and I love it. With and fixed stock it seems to balance really nicely, but I don't plan to be carrying it around or shooting competitions with it. My friend has the fluted 18" Sabre in a midlength, and it is a great shooter, lighter than my WOA and perfectly usable for 3 gun competitions. I don't thing you are going to benefit any from the 20" bushy, especially as it sounds like an hbar, just more weight than it's worth unless you plan to shoot only DCM type competitions with it. If you really want a 20", look at the WOA SDM barrel, a fluted 20" match barrel patterned after the Army DM-R. With the flutes I think it weighs about the same as the 18" SPR profile.
Of the above choices I would probably get the fluted Sabre from PK Firearms or the WOA SPR from ADCO firearms. I'm sure you could get either company to build the complete upper for you, and Steve @ ADCO builds some really sweet SPR uppers, but they aren't cheap.