I rendered a few more options. Maybe someone can share more insight.

215mm high gantry:
Option 1 (120x80 + 160x40)
This is the closest to what I currently have.
Not really much more expensive. Everything (motors, bearings) still fits neatly behind the gantry.
Click image for larger version.  Name: Gantry_1.jpg  Views: 1085  Size: 120.4 KB  ID: 29905

255mm high gantry:
Option 2 (160x80 + 160x40)
Fairly similar, but 40mm higher.
Click image for larger version.  Name: Gantry_4.jpg  Views: 1089  Size: 150.8 KB  ID: 29906

Option 3 (120x80 + 120x80)
Using identical extrusions.
Click image for larger version.  Name: Gantry_2.jpg  Views: 1053  Size: 141.7 KB  ID: 29907

Option 4 (120x80 + 160x80)
The expensive option.
Click image for larger version.  Name: Gantry_3.jpg  Views: 1074  Size: 151.7 KB  ID: 29908