The right answer to your question is: You are talking about the same phone. There is no difference.
The difference between MK16a and MK16i is basically the 3g networks they work with.
MK16a:
850 MHz, 1900 MHz, 2100 MHz. (GPRS, EDGE, UMTS, HSDPA, HSUPA)
MK16i:
900 MHz, 2100 MHz. (GPRS, EDGE, UMTS, HSDPA, HSUPA)
Hope this helps