I have built both the 18 wayy sIII and the original 15 watter (TC 15 kit on the way!)
My experiences playing both amps over the past year (bar gigs and home playing).
-volume is about the same
- the 15 watter has a LOT more clean headroom.
- the 15 seems for me to take pedals much better. I use my 15 almost exclusively to gig due to the fact it, with my pedal board (a chorus, a few overdrives - tube screamer and a plexitube, delay and wah) it allows me to play anything from a chimy crystal clean Tom Petty tune, to anything in 60's/70's rock and roll up to modern rock with the plexitube pedal. The 18 watter gets too "fat" with an OD in my opinion. I love the natural OD the amp has on its own but I cant get it to sound the way I like with an OD pedal the way I can get the 15 to sound.
- I like the 15's tone (high, low) controls. Very responsive and depending upon what guitar I am playing it is easy to add or cut bass/treble and do it simply and quickly.
If I had to choose one, it would be the 15 for its versatility. But I LOVE my 18' as well and the best thing to do it build one of each!
I cant wait to try the TC15 though and the ef86 side of life
AJC
PS I also have the Deluxe... another completely different animal!