Difference between sensors

Hi everyone,

I'm new with the Arduino and I'm about to make a project: Botanicalls Twitter DIY.
The thing is that I wanna do this project but adding some features like measuring different things of the plant, so I will need new sensors, like a temperature and humidity sensor.

The thing is that there are a lot of sensors in the market, but as I am new in this, could someone recommend me these sensors?
For example, I've found these two sensors:

http://www.eztronics.nl/webshop/catalog/product_info.php/cPath/1_21/products_id/187

http://www.eztronics.nl/webshop/catalog/product_info.php/cPath/1_21/products_id/143

Ok, but what is the difference between both of them? The difference in price is huge but what is exactly the difference? Maybe one of them is already ready to work with just with Arduino and not having to buy any other stuff in order to make it work?

Sorry for so many questions, but I cannot find my answers anywhere.

Thanks a lot everyone!

If it doesn't sound daft, one is Temperature and the other Humidity/Temperature.

What are you doing with the sensor ? Just measuring Temperature or is Humidity worth the extra cost ?

Other methods are getting the data via i2c instead of analogue

Well I have planned to measure the humidity, the temperature and the PAR light (with a photometer, I guess) and the amount of nutrients the plant or the soil has (I really have no idea of how to do this; I don't even know if it's possible)...
What is the i2c, by the way?

and the amount of nutrients the plant or the soil has

That sound very expensive, if it all possible !!!

i2c :-

read about the wire library :-

It possible to get all sorts of sensors , light , Humidity , Temperature , eeprom , real time clock, lcd/led displays all running on just a 3 wire bus together.

Which can gives back digital information which can be easier to workout than an analogue value.

http://www.nuelectronics.com/estore/index.php?main_page=product_info&cPath=4&products_id=6

Try looking for SHT10 , that looks a cheaper sensor for Humidity / Temperature