What are the difference between nitro Rc and Electric Rc cars and what one should i get?

The primary difference is that the nitro car uses a fuel powered engine, is usually faster, and is noisier (IF that matters,) and USUALLY does NOT have the ability to stand still or go in reverse.
The electric uses a battery powered motor, is quiet, and CAN have a reverse easily since the motor will run in either direction.
They do NOT compete against each other. EACH type has it's own races of various types.

Hope this helps.
References :

Nitro is traditionally for those who want to do things very seriously. They take time, they take money, they take maintenance. For instance, i race my 1/8th rallycross buggy every week. For a days racing, in which i'll use the car maybe two hours, i will need to spend at least two hours cleaning, oiling and fixing it, minimum. Much more if its muddy. Nitro is also quite expensive. For instance, your cheap nitro 1/8th buggy will cost maybe $250, plus $15 for a failsafe, plus $25-$35 a month of fuel, depending on how your car is tuned, the fuel, and weather conditions. This is without the inevitable spare parts you'l need to buy.
Point being, nitro is definitely the hobby side of things.
As to electric, there are really two types of electric now, brushed and brushless.
A brushed electric can cost maybe $100 RTR (RTR=ready to run) plus perhaps $30 for a good charger and $25 for a decent battery. They are the slowest of the rc cars, however, i run a brushed electric in the '2wheel drive 1/10th' class, and regularly get round quicker than nitro rustlers, firestorms etc. Brushed should not be dismissed due to it being old hat, it's cheap to run/spares and a good laugh. Clearly, you will never keep up with a good driver in a nitro of some sort, but they are a very good first platform to learn the basics of driving, and decide wether this is actually the hobby for you. (I just bought the ansmann 'macnum' kit, this along with the 'madrat' are really very very good for the price they ask, available in either kit or RTR, kit will probably teach you more in the long run.)
Finally brushless. Brushless motors are fairly new to the scene, and are actually pretty immense, IMHO. Brushless motors are around 95% efficient, compared with brushed motors around 45%. This means they can push much much quicker and deal with much higher current before heat becomes too much of a problem. Brushless is as quick as nitro, though some will disagree, and has a very distinct driving style. However, Brushless rigs, and the li poly batteries that power them are as expensive, is not more so, than nitro.

The last thing i will add, if i may, is that economy here is a false economy. Cheap RCs break, break and then probably break again. Get the best you can, or save up some more cash, it will save you time and money in the long run.
Check out;
HPI
Losi
Traxxas
Thunder tiger
Team associated
Ansmann
Are the brands worth a mention.
References :
Brushless losi 8ight, nitro rustler, ansmann macnum, maverick atom, thunder tiger sledgehammer.