Your systems will be different from mine, so your mileage may vary, but you'll see from my thread below that I've had serious issues with McAfee.
I'd suggest getting free trial scans, simultaneously, from each vendor you're considering. Add Qualys to your list, and if your SSL vendor or card processor recommends someone, or even better offers a discount, try them out too.
You should go with the vendor that finds the most real issues, after false positives have been accounted for. The aim of the game here is to be able to sleep at night, and not have nightmares about being pwned!
A VeriSign or TrustE seal is probably better known and better for conversion ratios than a "hacker proof" one - there lots of seals to choose from and only so many pixels to spare for these things before you end up looking like some guy at election time with too many buttons on his jacket.
Many people also know that sites with such seals have still been broken into, and there's always the chance that someone will your "hacker proof" button as a challenge and have a go at you on the strength of it.