I have a Zoom MS-50g with 3 presets programmed - clean, crunch and lead and hitting the button just cycles between them. Since it’s just a pedal I don’t think it can be any smaller and lighter as a backup. Obviously no amplification in there. It would go into a powered monitor that I assume that I could get hold of anywhere in case both the AFx2 and my monitor fails and then to FoH same as the AFx2.
I'm seriously considering picking up an iRig HD for my iPad and buying Bias and JamUp Pro. That combined with Garage Band and I'd have a great portable DAW for capturing ideas on the go.
Do consider the Apogee options too EJReally nice and whilst the 'basic' interface is great, look a little further up the range and you'll see they can charge the iPad whilst in use too
I'll wager that once you try JamUp & Bias, you might even consider using it for more than you initially thought...
Cubasis and Auria look interesting. I don't see ever completely converting to iPad recording, but for on-the-go stuff when I don't have access to my full studio, sure, why not?
I have been thinking about the OP's question and it's not specific as too the type of back up rig, ie direct or amp and cab required. I still like the TEC 21 offering but you need something to hear it with (amp and a cab). That said if you add their Power engine 112 or maybe the Electro Harmonix Magnum 44 power amp pedal and a 112 cab it would round out that rig nicely. The problem with that is it starts to make things not so cheep when you start adding that up may as well do a line 6 spider valve or the many other offerings in that same category.
For the direct route I think it would have to go the Line 6 HD for a plug in drop and go type of deal. Even though it's not on par even with a 1st gen Axe it's a lot more convenient as a back up rig that's fairly inexpensive.