I've got an iRig and while I've never used it for uke (or any acoustic instrument) I did not find it satisfactory with my guitar. It's pretty noisy, for some reason. The ampkit dongle is an active device and should sound cleaner but I steered away from that one at the recommendation of some friends who said the build quality wasn't great.
The device I've been keeping my eye on for this kind of thing is the Apogee Jam. It's been "about to ship soon" for a while now, current estimates are 7/29 ship date, but who knows. It's more expensive, but Apogee makes quality products.
I do like the Amplitube software, though any of the software should work with any of the devices, including Garage Band.