Keep the car, it makes the most sense financially, but remember to do your research when this one is worn out and you need a replacement.
If you don't get the HOV stickers where you are... stick with the regular hybrid. The cost difference would take years to recoup for the average driver. This is especially the case if you don't drive that much in the first place.
Unless you have cheap power or can get HOV lane access (if that's important to you), you're better off with the regular Prius.
And that one payment was almost ALL principal. =( You're considering spending thousands to save pennies. As long as 2 of the 4 people don't have legs.
Remember also, that in buying a new car, you've been charged a premium for things like warranty and cheap fixed price servicing. It would be silly to throw these away. In my situation I would wait three years for the engine warranty and $125 servicing to elapse then jump into a GEN IV.
For a thousand dollars a month, I'd say you did fine already. You had to be strict with your salesman or he would have crawled up out of his tank, started doing the upsell dance, and just purely slimed the **** out of you. You'd be driving around now in some $100,00 car with a vinyl top and "aggressive styling" and would be still making payments when you're ninety years old. Any time you buy a new machine you should immediately stop looking at ads for that kind of device. There will always be something bigger/faster/shinier in the next weeks ads. Don't fuss about it, you did good. I would suggest you consider the Pip as a spendid experiment, and all us Pip drivers as being your personal beta testers. Use what you already bought, hack it any damn way you please. Enjoy the heck out of it. When the time comes that you've worn out or gotten tired of your present car, by then pluggable/electric cars will have serious track records, lower prices, better reliability, and you'll be able to skip past the 2014's and straight into the good stuff...
IMHO You'll lose an easy 3k on a trade that soon.. My Buddy bought a new equinox 2 months ago and decided he couldn't afford the payment. Just traded it and lost 3600 on the car, and bought a used equinox. Same body style, two years older. He paid 24000 for the new one, traded it for 20,400 and bought the used one for 18k. Terrible idea on his part. Don't rush into anything.... Your not going to save that much in fuel to justify the loss.
I believe there are third-party options for mounting additional traction batteries, however, they take away a portion of the cargo space. I'm not sure a heavier vehicle with lesser cargo capacity is a step in the right direction. I'm waiting for the next big breakthrough in battery charge densities. The nanowire electrode holds some promise for lithium batteries. Researchers developing cheap, better-performing lithium-ion batteries
I would drive the prius and wait to buy the plug in later. If you drive 10k a year at 50 mpg that is 200 gal of gas, x $4/gal=$800 switching cars now will likely cost you 2500 and that is like free gas for 3 years. I vote drive the car you have and as others have said the plug in will be better in 3+ years from now. thats my .02
Let's put things in perspective: What did you give-up by not purchasing the plugin? 10 miles of EV mode. That really isn't a big deal in the grand scheme of the whole world (which seems intent upon creating WW3: Russia versus the EU) (plus ongoing killing by the US elsewhere). Or if you prefer to focus strictly on the world of cars: EV mode would have saved you a little bit of gasoline... about 30 cents a day. Wooo.
Actually it is a big deal for some people who have the right combination of circumstances. My situation is one such example, my trips are usually short and most of the time I make the whole trip in EV mode. My EV is running around 82% and my cost of electricity is about 1/3 the cost of gas. Not to mention the savings from driving short distances in cold weather where the ICE doesn't have to run but would not even barely get warmed up if it did. Plus in winter ICE mpg goes down more substantially then EV range. In the winter my 2010 prius only got around 30 to 40 MPG, actually sometimes even as low as 25 mpg. Then there is the increased fuel economy in HV mode when driving on longer trips. My 2010 would average 52MPG and the 2014 Prius plug-in averages 56 MPG.
I traded in a new Honda Civic Ex 3 months after I bought it (my son required a private school and 23,000.00 per year tuition was a big hit). The dealer completely paid it off. I was out nothing trading for a new base model Sentra that was 2/3 the cost. So if puts your mind at ease, check into your options. Other than that, it's just speculation. Not sure how the market is in NY but where I live they can't give away used hybrids with any amount of miles on them so time (and mileage) might still be on your side.