The XT Live is still a fine device, and I believe it's capable of delivering some great tones. (Disclosure: I have never owned/heard any Pod XT device - I am just going by what I've read here over the years).

However, in terms of being a backup to the HD500 I am presuming you would want something that you could easily use to prepare presets that sound similar to your HD500 tones. In that context I think it might take a long time to do the necessary tweaking on the XT, and you might never get close enough to satisfy you. You might find it easier to do this if you picked up a used Pod HD300/400. Those devices use the same amp models and FX as the HD500, and the presets, while not directly transferable, are much more easily prepared to match the HD500 tones as closely as possible given the device differences.