Mira is 660 cc and Swift is 1328 cc
On long route as per my experience swift will not give you bad average, it must be above 15-16 km per litter.
If your travel is on plain road then there is no issue in using Mira, but SWAT terrain is hilly where IMO Swift will perform better and will not disappoint you.
based on these figures you can decide better. choice is yours, you have to compromise on one thing, either Fuel or Comfort with Power