The P4 is on a completely different level of performance! Basically, you can make it sound like you want, due to it's full opamp costumization. So, comparing it to the E12 isn't really fair. The E12 is a great amp for the price, but it can reach the P4's performance level. I have my P4 with a custom opamp configuration to my liking and it's just awesome. But I have an E12 too and I use both, as it's a "fun" sounding amp... As for the size comparision, I use my portable amps with an iPod Classic through a LOD cable. The P4 is a bit short and bulky to use with the Classic and the E12 is a bit too lengthy (although it's width and height are perfect for the Classic)...
Thank you so very much! Yours was precisely the type of comment I was hoping for, and was consistent with my thinking. The E12 just seems, to me, to be a fun amp.
The fit and finish of a device that I hold in my hand is often very important (though not with the P4 due to its customization abilities.)
BTW, I ordered HiFlight's TopKit today.