The processor may be only 400mhz, however, it's a different animal altogether than the apache; mainly due to it being a dual core processor, basically, having a separate 133mhz core dedicated to the phone part.
The biggest advantage to the touch is by far it's memory... Here's your "big" reason to upgrade, big memory:
http://www.mobiletechreview.com/phones/Sprint-Touch.htm
"The Touch has 128 megs of RAM (used like RAM on your computer to run programs), with 73 megs free at boot (that's nearly 3x available memory on the Mogul and almost 4x compared to the GSM Touch). This means apps will run more smoothly and you won't have to quit apps to free up memory very often. We hope that all Windows Mobile devices come with this much RAM going forward! For storage, the Touch has 256 megs of flash memory with 151 megs free for your use to install programs and store data"