The regular iRig goes through the headphone socket but is subject to noise as well as some "crosstalk" from the outgoing signal. I have not used the HD but it is an entirely different design. There are several competing products including the Jam and iRig HD (other units be Line6, Sonoma, others I'm sure) that all cost about $100 and interface to the 30-pin or Lightning connector. Any of these that have the right connector (or adapter) for your ipad should be fine.
FWIW, I can also vouch for the Jam. I have used it to record all 3 of my ukes with pickups (all passive) and it works great.
Another thought if you think you might want to also record vocals (or anything with a mic) later is something like the FocusRite iTrack solo or iTrack Dock. I have the Solo and it also works great and is not much more than the guitar-only interfaces.