It depends on whether the A/D convertors are better in the Tampa or the Firepod. The XLR out puts an analog signal into the firepod, and the firepod must then convert it to digital. The SPDIF out is a digital audio stream that was converted by the Tampa A/D convertors. I can't quite remember it right now, but I think there's a gain stage or tube emulation or something that's bypassed when you use the SPDIF. Like I said, I'm not sure, but it's worth looking in your manual before deciding which way to go.
EDIT: just looked it up, the 20dB pad does not effect the SPDIF output, thus bypassing some of the "soft clipping" functionality.