Unfortunately, if your taxes are too complicated for the 1040EZ form, then your tax situation is effectively unique and you need to try both options and see for yourself which one is better.
If you do your taxes yourself, you may be more likely to do a more thorough job in digging everything up. You might even find that you can deduct some things that you hadn't thought of before.
On the other hand, whenever I've gone to a tax professional, it's always been pretty much an all-or-nothing proposal. You sit down with them and hand them your records, they ask a couple simple questions, and they either give you your completed tax return on-the-spot or they have you come back in a week for a brief review of the final numbers. If they don't prepare your return on-the-spot, you can usually send additional items later on if you think of something that you forgot the first time around, but for the most part it's still a one-time shot.
That said, I'm beginning to think the difference in monetary cost of completing even a mildly complex tax return is going to be insignificant, and the main factors to consider are the value of your own time and how much of the tax code you want to learn (because, in my experience, the software always refers to additional IRS forms or codes that are not automated in the software). In theory, your tax return should be the same regardless of whether you have a tax professional do your taxes or, if you do them yourself, which software you use. Given the same inputs, you should get about the same outputs. Even though that theory doesn't always hold exactly true, all the options should get you in the same ballpark--close enough that it doesn't make much difference in the grand scheme of things, unless your tax return is done incorrectly (e.g., you choose the wrong filing status or forget to take a major deduction).
Suppose you're married and you or your spouse is a partner in an LLC. Maybe a tax professional wants to charge you $500 for your tax return (this will vary based on your circumstances). You could alternatively buy the tax software for $40-$300 and spend 20+ hours navigating through the interviews and reviewing tax codes for the decisions and worksheets that are not automated in the software. Depending on how much time you personally have to spend on the tax return, one option might be better than the other. Maybe you have to pay your in-house accounting person to use the tax software, or you have to pay an employee to cover for you while you use the software. Keep in mind that the tax professional and the tax software are probably deductible, whereas your time may not be. In the end, even if you save money up front, it might be a wash on the following year's tax return, especially after you consider the uncompensated time that you could have spent with your family, on your business.