Small interface for laptop for live fx

[ubu 1]

I’m in the market for a small, cheap USB interface. I use a Macbook as a guitar fx unit live. I don’t run amp sims - I split the signal; the main signal goes into a Sansamp and then into one channel of a Fender Twin, the other via an interface into the laptop where I run Plogue Bidule. Or rather, that’s what I was doing with my previous (now expired) Macbook. My replacement (late 2009 Macbook running Mavericks) is USB only, so my firewire interfaces (a Saffire and and Echo Audiofire) are useless now.
As the laptop stuff is mostly for extreme FSU sounds and granular stuff, it doesn’t need to be brilliant in terms of fidelity. Obvious candidates are the iRig HD or Pro, Apogee Jam, Alesis Core. The tiny footprint of those units certainly appeals, but I’m dubious about using the Macbook’s headphone output straight into the amp (or into a DI linked to the amp). Doesn’t that introduce a ton of noise into the signal?
I also have a usb midi footwitch (one of those Hungarian-made ones you might have seen on Ebay, highly recommended btw) - would you anticipate any issues running something like the Apogee into one USB slot and using the other for midi? With something like the iRig Pro, would the fact that it is also a midi interface be a problem if I wanted to use the laptop’s second USB port for the footswitch? Ditto the Sonuus i2m - the audio to midi stuff might be fun, but I wouldn’t want that to interfere with the footswitch, and I don’t want to have to start messing around with hubs. Any thoughts, good people?