My take is that you keep the bars you like with the 26mm clamp and get a stem with 31.8 and use the shims mentioned above. It seems that 26.0 bars and stems are becoming more scarce all the time and you'll have more options of handlebars if you decide to change in the future. Also agree with others that larger clamp size isn't always stronger/stiffer. It's more dependent on the type/thickness of aluminum or CF used although with CF I prefer a larger clamp due to the fact that with a larger clamp area you need less clamping force and less likely to cause damage by over torqueing.