| I had the x3 Live Pedal, which is basically the guts of the Pro in a Pedal form factor.
Really loved it--there's all sorts of tones you can download, lots of amp and effect models, a gazillion routing options, and it's easy to configure with software you can download for free from the Line 6 Website.
FWIH, the Axe-FX is indeed a higher quality (and likely better sounding) unit, but whether or not the difference is worth the extra cost really depends on what *you* think. I'd try out your guitarist's unit (see if there's any bass presets on it) through your bass amp, and see if you can find any of the Pod X3 units and try them out for comparison.
One thing--what these monster multi-effects modelers do really well is give you a literal *universe* of tones to play with. If your group and playing goals involve lots of different tones, that's great. If you need only a couple good tones there may be better options out there.
I ended up selling mine because it was really more than I needed. Also, the X3L is designed to go into a mixer or a guitar/bass amp. The outputs on mine were underpowered for going into a professional power amp. The rackmount version *may* be different in this regard. | 
