I was in the same position about 8 months ago and made the decision to purchase a MacBook just to carry my OmniFocus GTD to work with me. It has worked great for me and was well worth the investment. I sync between iMac at home, MacBook at work and iPhone. You can read about my experiences with GTD and most recently with OmniFocus on my blog at
http://johnkendrickonline.com Good luck to you, John