The tutorial movie library has a couple good videos on the 4423 and 4425 performance.  The 3423 is mentioned many times in the 4423 video.  Over time, Pico made improvements to their software that helped the performance of the 3423, which is stated in the video.  There's a 3423 sample rate chart in a forum post from 2008.  

It would be a very large jump in performance from a 3423 to a 4425 or 4425A.  Sample rate, ground offset, need for attenuation at 50V vs 200V are some major differences 3423 vs 4425/4425A.  Sample rate will effect how the scope can be used, this alone would be enough to justify the expense of a 4425/4425A.  The 4425A performance is the same as 4425 but the 4425A adds features such as powered probes to allow graphing of resistance, automatic probe type detection, auto-zeroing current probes, no batteries in current probes designed for the 4425A.  If the PC has a USB 3.0 port, the 4425 and 4425A can take advantage of that for fast file transfer. The 4425A is expected to utilize this 3.0 port for future probe development.  The probe improvements is reason enough to consider a kit rather than a 4425/4425A scope only.

For me big advantage for 4425a is differential channels and power supply over BNC, no longer need to wear spare 9v batteries