JShelter isn’t mainly for spoofing, it’s about blocking a bunch of potentially harmful advanced javascript features, often used for tracking. Any spoofing is mostly to keep sites working with the missing javascript features.
I have it installed on one of my browsers. I wouldn’t recommend using it unless you’re willing to tweak the settings for new sites you visit, because I’ve had it break sites pretty often with the default settings.
I haven’t used Chameleon, but it seems to do some stuff like change the user agent that JShelter doesn’t do. I’d assume it’s more useful to get around a site designed for a specific browser or operating system.