Digital BW, The Print

I am using the new Nik sharpener pro 2.0 plugin for printing and it now 
has settings to match the different resoltions used by the printer. I 
have the R2400 printer and usually use the "best photo" setting which 
Epson tells me is 2880.

The chioces I have in my sharpener software are 2880x720 and 2800x1440.

What does the second number mean in these designations and which would 
be the appropriate one for my above noted printing preference?

Thanks for bringing me up to speed. I never did understand that second 
number...

Scott