If the POD Express Guitar had a truly clean channel it would be a complete solution for the tones I want, which would be incredibly convenient given how small it is.
Listening to the demo of the POD Express Black, it sounds like it has a very clean amp model called "Pure". I would happily have traded away one of the other amp models in the POD Express Guitar (in my case the "Lead" amp model, but I'm sure others would fight for their own choice) to get an amp model as clean as that.
The POD Express Guitar was my first Line 6 unit and I was very impressed. I've ended up buying a second hand X3 because it had a huge amp list including many bass amps and I know I can resell it for only a small loss if I don't get on with it. I love the Line 6 "Clean" amp model. It really warms up when the drive is increased while staying clean. Unsurprisingly given the age difference, I prefer the POD Express Guitar's amp models over the closest equivalents in the X3. I'll be interested to see which I end up using more.