reverse parking sensors or reverse camera??

i want to go for either reverse parking sensors or reverse camera for my linea.

Which one of the above 2 s a better option?
Should i install both of them?
which brands are a good option?

These two are completely different even if their purpose is the same - help you reverse better and without bumping your car anywhere. The reverse parking sensor will give an alarm if it senses any object within a specific distance of the car. The reverse parking camera on the other hand will show you the area at the back of the car on a screen like the rear view mirror. I would suggest you try and use each of them probably on a test drive vehicle or a friends car and go for the one that you are most comfortable with.

how much does a park sensor range cover. does it ditect objects which are low, may be those which will hit the lowest part of rear bumber

to be frank I am not aware of the range but I think it should detect all the objects while reversing.

Sensors sense anywhere between 1.5 meters to 10cm, depending upon the brand and the specification. Sometimes, due to the wave propetry, some small objects cannot be dected. During rains also, some of them malfunction. You can opt for a camera instead IMO. The camera covers the area which neither mirror can, and can help you out with small things like a pavement, a small pole sticking out of the road etc. Installing both will be a mess on your bumpers, except if you can get body coloured sensors.

A small doubt...is the small round circles I see on the bumpers of some of the luxury cars the parking sensors? are they visible?

Yeah. They are the parking sensors. Luxury segment cars have it mounted very well within the bumper, with a small black circle seen. But aftermarket sensors are 'clipped' onto the bumper with a hole which make them stand out. 

i own a fiat punto emotion pack does installing park sensor from outside will void my warranty. as it is fiat and A S S is a problem i donot want to mess up with warranty. please clarify

You are talking about Fiat here! So please check if there is any clip on kit available. Usually the reverse light wire is used to power the parking sensors. So, if the aftermarket guy cuts the wires to speed up the job, it will throw the warranty out and high. That too, fiats do visit the ASC a couple of times for warranty jobs. So this could lead to the warranty being void. Check out with te dealer itself.

The dealer offers me an option of having reverse sensors with display at a cost around 4k after all discounts. I hope it will do the job. Is it worth 4k. I was keen about outside market because i have a client who can provide me a huge discount. The question is that is it worth 4k. What would be the outside market rate of a good brand with display.