If you want to plug your guitar straight into garageband for the virtual amps, there is no point in wanting a stereo input. Your guitar is mono (well, most likely), so unless you use real physical stereo pedal effects such as stereo chorus or flange, just take your 1/4 to 1/8 adapter and plug it. I've been doing this for a long time and it is the cheapest solution that still gets the job done. Of course, for pro recording, you need a sound card.