I just bit the bullet and ordered the single blade Agbay that can level cakes up to 6" high and tort cakes up to 20".
Even though I hobby bake and give all my cakes away for free, I still want them to be absolutely blow-your-mind-killer-good cakes inside as well as out. I've wanted an Agbay for 2 years and finally said, "Screw the credit card balance! I'm gettin' one!". (I'm sure that Suze Orman would be very disappointed in me.....)
I use the "level in the pan by slicing off the top" method for the tops of my cakes. Here's the method I was using with fairly good results to tort cakes:
"Using Wilton's new Cake Marker [and Sweetwise's "lock-arm technique"] to achieve level, torted cakes!"
My knife is a 12" bread knife from Winco. You can get them on Amazon.com