A fish only or FOWLR system is not too different then starting up FW. Just a bit more expensive. Now if youre thinking about a Reef System, well then my advice would be to start out with the FW system. Cycle the tank properly, then get yourself into a religious maintenance schedule (monthly water change, regular testing, and routine equipment cleaning) and save up your money in the meanwhile. If you can stay on that maintenance schedule for 2 years without stray, and you have a few grand to spare to sink into the proper equipment and lighting, then think about starting the SW Reef. In the meantime do alot of reading (books and internet) on the care, equipment, and maintenance involved in maintaining a reef system. Ask lots of questions in the SW forums. And get yourself a solid gameplan (ie what corals, inverts, and fishes you would like to keep) and make sure to check compatabilities.
If you follow this advice, when the time comes to go reef, you will be ready to do it right the first time, and you will find that it's really not any "harder" than keeping FW. It just requires more money, a bit more time, and a lot more commitment.