Hello Richard. Lots of good advice that you're getting.
I just recently added a 12mm T4 to my collection,
and liked it so much, I just bought a 17mm.
Haven't felt the urge to get the 22mm yet.
My longer focal lengths are T5's (20, 26, 31mm).
I had held off on getting the 12mm, since I still carry
the Meade 14mm and 8.8mm UWA's in my case,
though they've been getting a lot less use.
Once I go to more power than the 12mm provides(150x),
I generally go for the T6's (11, 9, 7, 5mm),
unless I'm doing planetary viewing.
Anyway, I would definitely recommend the 12mm T4!