Maintenance cost between the two? Not much. Obviously the initial setup would be more for the bigger tank but after that then there wouldn't be much difference. Keep in mind that the bigger tank will cost much more especially when it comes to the lighting. Honestly, if you want a real nano then I'd go for the 14. 29 always seemed a little big for it to be called a nano IMO. It just seems like a small tank (yes I know that a nano is just a small tank, but hopefully you know what I mean)
The water chem will swing easier in a 14 though. Just food for thought. 
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55gal,216 watt Tek T5, 45#LR, 45#LS, 3 maxi-jet 900's on a natural wavemaker timer,phosban reactor, 5 gal fuge (built out of standard 10 gallon tank) with chaeto and feather caulerpa, sealife systems protein skimmer, wet/dry, 3 blue/green chromi, 1 skunk cleaner shrimp, 1 coral beauty,1 scopas tang, 2 false percs, 5 mexican turbo snails, assorted dwarf blue leg hermits, astrea snails, and nerite snails |