Definitely Ampkit Link. The most important distinction is that Ampkit is an active unit, with a preamp inside.
The iRig does a good job of impedence matching, but that's only one part of the equation. Because your iPhone headphone jack needs to send 3 signals (stereo left/right, guitar in) but has only 1 earth between them, you will get crosstalk which results in greater risk of feedback. There's an anti-feedback setting in the Amplitube program, but I think it uses some form of phase cancellation to eliminate feedback. This also results in a significant change to the tone. With the Ampkit Link, the preamp helps improve this significantly (but does not eliminate it entirely)
Software-wise, you can use either of them (I have had good results with Peavy Ampkit, but Ampiltube sounds pretty good as well... your mileage may vary). Probably best to download both free versions first while you evaluate. The new Amplitube has some very nice features though!