I know how difficult these decisions can be without the benefit of hands on testing. I have both the 100-400l and 150-500OS... here's my take:
* Both take very good pictures, although the 100-400l maybe a touch better in color/contrast.
* 100-400l is push/pull and 150-500OS is twist zoom which makes it a personal preference thing more than functionality.
* The Sigma OS works very well and is on par with the Canon IS.
* The 150-500OS does weigh more than the 100-400l… but again, seems to be more of a personal preference thing since the weight is balanced well on both.
If it helps you at all… I’m only 1 ½ hours north of you so if you want to try them both out and have some tactile feedback before making your decision, just let me know. I’m usually in Harrisburg weekly but this week I’ll be in Pittsburgh the first half of the week.
If you're considering upgrading your body I can also let you try out a T1I, 7D and 5DII for comparison... all are gripped because the extra size/weight is a benefit to me and my large meat hooks. Just bring your memory cards and then you can peruse the image files in your leisure to see which works best for you.