Hmmmmmm, well, I can only say that getting clean natural sustain is difficult to do in general without volume. Touch and the set-up/resonance of the guitar have a lot to do with it as well. Practice on technique and precision will help but there's a mechanical side to it as well. Compression effects will help to give greater sustain (at least you'll perceive it as such) at the expense of dynamics but the nature of the digital beast is that at some point there will be an unnatural cut-off rather than a slow decay that we translate as sustain. Tube amps will naturally compress giving you sustain at higher volumes and distortion as well as sympathetic feedback will also create greater sustain, but that's not clean tone (what I think you're asking about). If you're trying to get the sustain direct out of the Pod, there's not a whole lot you can do without adding gain or volume that dirties up the tone. If you're running the pod into a power amp, upping the volume on the amp will help but eventually you'll still lose the signal when you hit the volume threshold on the processor.
As for the Spider, considering that it's made by Line6 and I'm pretty sure it's guts are similar to the Pods only fewer in models, will it really be that different than what he's already got?