Yes, I plan to use software instruments entirely. This helped a lot. I am certain the iMac version of Logic would be best. As you said, processor speed must be considered. Plus there is really nothing the iPad version can do that the iMac version of GarageBand or Logic can't surpass.
I think I've actually gotten to be pretty good at tweaking the software instruments in GB to a sound I am happy with. Guitars are a little harder but I think I'm getting there. The trick I think with guitars is sometimes they sound real and other times they don't. So what I do is stay away from the notes that sound electronic. Of course, my ear may not be as refined as yours and I may discover in the future the guitar sound is lacking. But until then I have fun coming up with different riffs and just getting better.
Thanks for the help. 
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