Re: Simms Headwaters or Orvis Silver Sonic Waders
The only real possible advantage I see in the Simms over the Orvis is the whole Gortex thing. Having the headwaters, I can definitely say that they breathe really well and keep me rather comfortable in the warmest of conditions. I am not sure how breathable the nylon is on the Silver Sonics - it could be just as good for all I know.
Outside of that, I don't see any real benefit to the Simms over Orvis. You have a shop close to you that can service and exchange instead of having to wait a few weeks to get any repair/replacements done. You can also actually try them on before you buy them to make sure you get the best fit. The Orvis ones have more features than the Simms (I especially like the waterproof inner pocket), and the Orvis ones are cheaper.
If I were you, I'd be going Orvis all the way