Frontier Airlines EarlyReturns - A319-112 vs A319-111 ?

Quick question to anyone who knows:

What's the technical differences between an A319-112 (like N910FR "Sal The Cougar", which I flew on on Monday of last week) and an A319-111 (like N944FR "Alberta&Clipper Polar Bears", which I flew on last night)?

The only differences I noticed as a pax was that N944FR had a swankier interior- focusable LED seat light bulbs above each seat, easier to turn air vent controls, etc.

I know what the first 4 characters mean (319-1__), but those last two chars seem to change between airlines and even between aircraft within an airline. This also is true for Boeing planes ie: (737-7__).

The difference between particular aircraft such as the A319-1XX and/or the A319-1YY is the engine that is equipped on the bird.

I'll take an example from your OP, using N910FR, which is an A319-112:

1.) The first digit (1) is version of a/c, (ie: A320-100 or A320-200)
2.) The second digit (1) is the engine manufacturer for said aircraft. For Airbii, (1) is the designator for CFM engines.
3.) The last digit (2) is the actual engine type, which in said example would be a CFM56-5B6/P engine.

The above example only works for Airbus a/c. With Boeings, the last two digits (ie: Boeing 737-297), is the customer designator, which, in the example to the left, would be for AQ, whose Boeing customer designator code is -97.

A decent reference:
http://www.rosboch.net/aviation.htm#AirbusVersions
(http://www.rosboch.net/aviation.htm#AirbusVersions)

why is frontier buying planes with different engines? seems that would drive operating costs up

All F9 A319's have the same CFMI engines. The only difference is a software upgrade to give ten (10) of Frontier 319's a higher Maximum Takeoff Weight (MTOW) using higher engine thrust. These would be the A319-112 aircraft. MTOW is 166,477lbs and max engine thrust is 23,500lbs on the -112's. These planes are also considered "overwater" aircraft; not ETOPS, just overwater. They primarly fly Mexico and Alaska flights.

so from a maintenance perspective there are no differences?

They are the same engine...again, it's strictly a software upgrade F9 had to pay Airbus.