Well, they're both big & heavy. If you can live without focal lengths below 24mm, and think you have FX in your future, the 24-70 is the logical choice. Also consider the Sigma 17-50 f/2.8 OS, which will cost less new than a secondhand 17-55, and is more a DX size & weight. See
Thom Hogan's review. You might also be interested in
his DX Zoom Lens opinions
Chris