Logic, Sampletank, and/or GarageBand for Ipad?

[Shankapotomus]

I'm not familiar with most music software and was wondering what should I get if I want to use the most realistic sounding software instruments?

I have GarageBand for iMac but I wonder if the touch screen version for iPad has more capability sound wise that I'm missing on my iMac?

Then the pros say it's really between programs like Logic and Pro Tools and GarageBand is just a toy, not a real program for recording or mixing.

However, I've heard the online demo of SampleTank and am really impressed with the sound quality. Sampletank has an iPad version so I'm wondering, if I want realistic sounding software instruments, if I should just cut out the middleman and go that route? The reason people recommend Logic or Pro Tools is usually because you can play more and alter the sound. But if I'm already getting a great sound from SampleTank on iPad, do I need Logic? Can I get away with GarageBand to record a decent CD? Or as my ear learns more am I going to want the tweaking power of a Logic Pro later?

I'm basically dealing in the rock genre sound so I'm going for a heavy rock sound from software instrument. I hope helps gives people enough of an idea that they can recommend one or the other from my choices.

Thank you.