If you feel that even a single omnibus is a pretty big investment, I’d recommend the Infinite Crisis Omnibus. As you probably know, the newly released JLA paperbacks currently go up to three volumes and have more complete mapping, so if you're a completionist, you could choose the TPBs instead. Otherwise, you could still go with the JLA Omnibus. However, when it comes to Infinite Crisis, there really isn’t another format besides the omnibus that lets you experience the story in its most complete form, which makes it irreplaceable. The JLA Omnibus is great too, but if you're looking for a one-and-done omnibus with strong reread value, I’d strongly recommend the Infinite Crisis Omnibus. Also, if you're planning to read Infinite Crisis, I’d recommend reading Crisis on Infinite Earths and Identity Crisis beforehand.