The interweb uses sRGB as default so that's probably the best to use if you want that. I have all my software set to AdobeRGB, including my camera, because ultimate print quality is what I'm after. Having said that this colourspace thing has always confused me and so I wouldn't be surprised if I'm also getting it wrong. All I can say is that on my machine there's no discernible difference between IQ, no matter how I view the image.
There's definitely scope here from a masterclass in colour by one of our more accomplished members. I'd certainly subscribe.
I've just checked my Lightroom preferences and you're right, there is no means of changing the colourspace within, only for exporting images to external editors. Seems a bit odd that but to be honest I've never really got along with Lightroom. I prefer my combination of ACDSee, Capture NX & Photoshop CS3.
Various lenses, SB800 & Manfrotto 190 with 460MG head