I'd go for force carbonation in many instances over natural. A rig will cost about 100. More if you want a 5 gallon keg to store the soda in. 100 if you want to carbonate directly into a 2 liter bottle. Less if you want to build a few components yourself.
With natural, you have to be aware of the sugar since yeast can continue to eat the sugar so either glass or plastic can blow.
With a rig like this
, it's pretty easy and takes 2 minutes to carbonate a 2 liter bottle, at approximately 8 cents per 2 liters. (I went nuts on math calculations earlier to compare costs and payback periods). If you had a soda keg, then you just use a keg instead of the bottle. I don't think I'd want to do all the shaking and I'd probably leave it a few days to force carbonate though. (Lift 5 gallons of liquid and shake for several minutes? I think I'd throw my back out again.)
Some things may be better with yeast, and some may not, I think it would largely depend on the drink. Whenever I visit some place, I just bring the tank and empty bottle and just keep filling it up. Went through 10 or so liters over this past weekend and just used some flavored syrups and maybe some chocolate syrup and ice cream for a chocolate soda/float.