Hi Curt,
The 12mm Nagler is a very good eyepiece. You will be very happy with it. It is easy to use, has long eye relief, is very comfortable and gives excellent views. The reason people sometimes choose the 13mm Nagler T6 over the 12mm Nagler T4 is because it is much smaller and lighter and has marginally better EOF performance. I prefer the 12mm Nagler T4 and in fact have 2 of them
Cheers,
John B