heres my take and opinion.
RO/DI water is the only way to go. Mine tests out at 0ppm, so the theory of distilled water being more pure IMO is garbage. As for buying RO water in a fish store, i believe my store sells 1 gallon for 50 cents.
Now, seeing you have a 14 gallon tank, we will say 10 gallons of RO per week. (evaporation and 1x water change) This is $5.00 a week. Which a yearly cost equals $260. (52x5) These are rough estimates but im sure it wouldnt be to far off. For the record, the filter of a RO unit does not need to be changed monthly. You can find a very nice unit for $200 that makes RO/DI water in your home which saves the trips to the fish store which also leads to picking up other odds and ends and gas and so forth.
Having your own unit allows you to know what exactly your putting into your tank. I have heard stories of fish stores selling tap water as RO, selling RO water from a RO filter that has not had the filters changed out, and pre-made saltwater that contains phosphates and/or ammonia/nitrites/nitrates.
Correct, RO units can go slow but you are supposed to pre-make your water and pre-mix your saltwater (atleast 24 hours in advance) however you can either get a larger unit thats rated more gallons per day or a booster pump that increases water pressure.
Another benifit of having your own unit is for when you upgrade theres no issues of needing water.
I wouldnt use tap water regardless of my location in the world, and seeing your located in NY, i would def. avoid that option at all costs. Alligators may come out of your faucet.
If we break down the price. (estimated)
$200 for RO/DI unit
$50 for bucket of salt
$20 power head for mixing salt
$20 heater for salt
$10 for container to mix in
we will add in 1 extra set of filters, $75
This total will make 160 gallons of SALTWATER. ( the RO unit will not need replacement filters for a long while, really it depends on how many gallons you make with it and how dirty your water is to begin with)
Now, store bought RO @ 50 cents per gallon
$80 for 160 gallons
$50 for bucket of salt
$20 for power head
$20 for heater
$10 for container
Store bought saltwater @ $1.00 per gallon
$160 for 160 gallons
i did 160 because thats what a bucket of salt is rated for, you now see a unit is expensive for initial purchase but pays for itself in 1-2 years and most def. pays for itself faster if you have a larger tank. you can def. find a unit cheaper then 200 and salt cheaper then 50, this was just an estimate to compare. As for the RO units waisting water to make water this is the method i use.
My unit is in my laundry room, when the time comes to make water (which is usually every other day or so) i take my container and set it the sink thats in the laundry room. i place the output water in the container (which is an empty 5 gallon drum) and the exhaust water into the washing machine. I forget about it for a short time, come back check on it, forget again and come back to a full drum and a full washing machine ready for laundry. *KEEP IN MIND*
if planning to use this method nothing is stopping your washing machine from over flowing. The exhaust water is perfectly fine to use to wash clothes so in the end i am waisting 0 water as i would have filled the washing machine with tap anyways.
Now as for distilled water, yes you can use it. no i wouldnt. to start i have heard alot of this water is stored in copper holding tanks, which is similiar to water passing through copper pipes and will cause issue of keeping inverts. buying distilled also will add up over time. if you think about it, a gallon or so of evaporation = another gallon of water to be replaced.