AVR-Chat

I am looking for opinions on JTAG vs DebugWire.

I have an AT90USBKey which hosts an AT90usb1287.
I would like to do single-step debugging on it.

I have found 2 programmers that claim to support it:
1. Atmel JTAG Mk II   at about $90 for a clone
2. Kee Electronics AVR Mk II Lite at about $48 
http://www.keeelectronics.com/AVR_debugWire_ISP_v2.htm

The JTAG Mk II can debug my chip using JTAG.
The Kee product says it will debug my chip using DebugWire.

What are the reasons for me to get one vs the other ?
Beside, of course, $42 :-)

Thanks!