Let me preface this by saying I can't really help you on the software side, since I work on a PC and therefore have no experience with Garage Band or Logic. For my Digital Audio Workstation I use Reaper, which is a great product for a fair price (it's also available for Mac). Also, if you really want to create songs with full arrangements, stick with your iMac over the iPad. Especially if you plan on using Virtual Instruments, which can be pretty processor heavy, and if the instruments work with samples you're going to need plenty of disk space as well.
I took a quick look at Sampletank and it is a sample workstation. However you will probably need a host application to act as a sequencer for it (to tell it which notes to play at what moment). It says that it works as a standalone as well, but I think it would then be in conjunction with a keyboard hooked up via MIDI; but in that case it would be for live use, so if you want to record the output you would still need a program to do that, and at that point you might as well use Sampletank as a plugin in a DAW.
The question I wanted to ask: Do you plan on entirely building your songs with virtual instruments and/or samples? I also do rock music and use a virtual instrument for the drums, but I play the bass and the guitars and then use a software amplifier emulation to get the sound I want. I've yet to come across a convincing virtual instrument for guitars, so (if you can) you're better off playing them.
I don't know if my response was at all useful to you, but if you have any other questions let me know.