Yes, I would go with the adjusted gross income, which I believe is line 4 of the 1040EZ return. For my I-864 I ordered Tax Transcripts, and if you are in the U.S., I believe they will email them to you... but you need a U.S. based cell phone to do that. I'm in Thailand, so it wasn't an option. If you order those by mail, it is straightforward and you get them in a week. On the standard 1040 you would use line 22 (total income)... which in my case is the same as line 37 which is adjusted gross income.
I know the I-864 is confusing, but not too bad if you live in the states as your mailing address, residence address, and country of domicile will all be the U.S. All they want to do is to make sure you earn enough. Also on that form, don't count your spouse twice as you are only sponsoring one person (later they will ask to to list your spouse, but will also say to not list the same person twice, so that was a little confusing). If you do click them twice, then that will change your household size to three instead of two, which will mess up the whole application.
Save your money and do this yourself... if you are at the I-864 stage, most of the hard work has already been done.