http://howto.homedepot.com/videos/wa...01/Echo-GT225-
Entry-Level-Trimmer.html

That's my two-stroke 21.2 CC grass trimmer.

I'm replacing the outer shaft and the inner driveshaft.

The outer shaft is easy enough.

The inner driveshaft will probably be either of these two...

Aluminum 6061-T6 Bare Drawn Tube
0.375" x 0.065" x 0.245"

Stainless T-304/304L Seamless Tube
0.375" x 0.065" x 0.245"

If steel should be used, should it be lighter than that?

I'm planning to use bearings in the shaft. The original flexible
driveshaft was contained within a long continuous (maybe HDPE, or
UHMW) bearing along the entire curved shaft.