Why would some flights use props instead of jets? Two years ago I flew from Tel Aviv, Israel to Tromso, Norway via Riga and Oslo. The Tel Aviv-Riga leg was flown in an A320, and the Riga-Oslo and Oslo-Tromso legs were flown in prop planes, Fokker airplanes I think.

What considerations are used to determine what type of plane will fly for a particular journey?

Obviously the choice would be limited to whatever aircraft the carrier has available, but given reasonable choice of any aircraft that seats the required amount of passengers, what considerations are used to decide on props or jets? Runway size? Fuel efficiency for short / long trips? Weather or temperature (both current and typical)?
I believe this question is answered here and a nice comparison can be found here but the short answer is efficiency. If memory serves jets gain their efficiency at high altitude moving fast and generally over a long distance. If you are going on a short hop turbo props will be more efficient overall. This chart provides a nice outline of the difference in efficiency. 



As for large piston pounders I am not sure of the efficiency but I would think the cost of keeping one running is high simply based on their age (no one is really making big piston planes anymore as far as i know). History channel has a nice show "Ice Pilots NWT" about a freight/passenger airline that still runs DC-3's and the general issues they have with them many of which is related to finding parts. 

That being said not all airlines had the money to jump right into jets when they came out. Lots of turbo-props are still flown simply because there is no reason to replace them.