James, Brisbane Australia
Modelling AT&SF in the 90s
The GP-9 will take sharper curves and switches , and the SD-45T runs faster.
The SD-45T was primarily a Southern Pacific engine wheras ALL railroads used GP-9's or 7's.
The 6 axle engines have more 'work's so should cost more. They also prefer 22"radii and #6 turnouts to run on.
Shawn,
Having bought locomotives from both manufacturers, I can honestly say that you won't go wrong with either decision.
Athearn RTR is almost as good as Proto. Especially detail wise.
If you want a 6 axle loco, go with Athearn.
Gordon
Brought to you by the letters C.P.R. as well as D&H!
K1a - all the way