I have gone the route you discussed--I started FW
and now have a 29 gal SW
that has been running for about 6 months. I won't be saying much more than the fact that I agree with Lando, but I will make a couple of points. In terms of maintainence time, I would not say that my FW
29 is all that much different from my SW
29. The main differences are that you have to mix up the water before the water change, that you have to add water each day to make up for evaporation which will lead to a change in the salt concentration, and there is more algae growth to scrape off the glass. In my experience, all of these combined do not constitute a tremendous difference. I do however spend a lot more time staring at my SW
tank watching all of the various critters going about their business.
Equipment-wise, the main difference is that the primary source of filtration would ideally be live rock and a couple of pumps in the tank to move the water about. This route is more expensive, but much more effective than going without. It also adds to the enjoyment value as you will find all manner of life forms coming off of the rock. Another side benefit of this is that the cycling time using live rock was much much shorter than the time it took to cycle my FW
The two biggest drawbacks of starting with SW
are 1) that there is simply more to be aware of and 2) that you're original setup and the mistakes that will invariably happen will be more expensive. For the record, I would estimate that I spent about $750 (rough guess) to setup my SW
tank before adding fish.
Just to offer food for thought--one of the coolest things about FW
though is that it is possible to breed many of the fish available. Good luck and keep posting questions!