Sigma 50-500mm f/4.5-6.3 DG OS HSM (renting vs. my 100-400mm) for day trip

[ddt0725]

Hi All -

I am planning another day trip out to Horicon Marsh in about 2 weeks and can't wait! When I was out there before, all I kept thinking was next time I need to bring a longer lens since most of the birds that hadn't left yet for the winter were quite a distance away (pelicans, egrets, herons). Now, going early in the spring, there will be more songbirds in the trees somewhat closer but still the farther distanced birds.

I am wondering if I should once again just go with my Canon 100-400mm and crop the photos when needed or rent the Sigma 50-500mm f/4.5-6.3 OS HSM for $100? Will I really notice and benefit from that extra 100mm? According to Roger's take at LensRental, the Sigma is pretty sharp but hmmm, sharper than the 100-400mm (cropped)? Plus, it weighs a pound more than my Canon. Will I really feel that added pound? I have been suffering for a month now with severe lower back pain and was diagnosed with spinal arthritis but the meds the doc put me on didn't help a bit so it could be something else.

Being unemployed, I can't afford to rent the Canon tele's especially with gas prices being $4.10+/gal. Gas prices alone almost cancelled my thoughts of going!

Suggestions and thoughts greatly appreciated!

Denise