It all depends on what you need to get out of the experience. Below are a few things to consider as you decide between short film and full feature.
1) Do you have a great short script and full-length script to choose from? Whatever you make needs to be based on a strong script. So if one is stronger than the other, go for the better script.
2) Are you trying to show off your creative skills as director or writer? This is a tricky area. Great shorts can get you noticed but there is a ton of competition and length matters. Even if your short is amazing, most festivals prefer programming the 10 mins or shorter films. This is because most shorts are programmed in a block of short films, which means longer shorts need to anchor the shorts programs. Your short better be brilliant in order to get a prime anchor spot.
So if you have a short that can show your brilliance in 10 mins or less then you may want to choose to make a short. Or you may want to decide to go full feature. Or take your chances that your 20 mins short will find its home and get you the attention you want.
3) Are you wanting to make money from it? Everyone wants to make money from their films but if that is your primary goal then you should probably make a full feature. The buyers' market for short films is small unless you are making one based on a popular topic or person. Shorts can have their own markets for sales if they focus on stories with built-in audiences (this is true for full features as well).
4) How much money do you have? If you only have 2 bucks then you need to make a short film because you can't make a feature on 2 bucks -- unless you have someone donating everything to you. If you have a few thousand dollars then it is possible to squeak out a full feature.
Again this is where things can get tricky. You don't want to make a full feature that really needs $100k for $5k. The only time it makes sense to do a full feature for $5k is when it is written for that budget and it's GOOD. I have seen so many films shot in one room that have bored me to tears. Just because you have a script set in one location and you can shoot it super cheap doesn't mean it will work or should even be attempted -- which leads me to number 5.
5) Be realistic. Take the time to seriously evaluate your goals and reasons for deciding to make a short v. a full feature. Don't make a quick decision based on emotion. Most people want to make full features but if you are at a point in your career that a short makes more sense, i.e. you need more experience, then make a short for the experience. And remember, you need resources to make any movie. Don't shortchange your full feature when you really only have resources for a short.
6) Timing. Shorts can be created and finished much faster. They can be uploaded to the Web in a jiffy and go viral rather quickly if you have a topic of interest. So timing can play a big factor in your decision for short v. feature.
7) What is the topic best suited for? Some topics may be better as a short than a full feature. Kitties and their antics do very well on YouTube in quick spurts. You definitely don't want to make a full feature of cats playing. You get my drift. Decide what format is best for your story.
8) Do you have connections to crew who can commit themselves over the longer timeframe of a feature? Connections to crew are very important. It can be very disruptive to the creation of a film to have crew coming in and out all the time. You need a core group to be part of your crew that will get you through the long haul -- at least a producer who can help replace those who can only do short spurts of work. Some stability needs to be in place. Until you have that stability, you should make shorts and build up your connections.
9) Awards. Don't worry about awards. Make a great short or full feature and awards will become a reality.
10) What is your end goal? Do you want to be known for short films? Then you need to make a short obviously. Do you want to make a great feature and you feel you have enough resources and the experience to make it happen? Then a full feature may be right.
In the end, it comes down to evaluating your resources, goals, quality of projects, and experience. If you are still having trouble figuring it out then you need to do more evaluating. Write down the pros/cons/resources for each and eventually the answer will appear. Trust your instincts. You usually know when you are making the right decision.