I have canon 500D with 18-55mm and 70-300mm lens. I have lot of interest in landscape and Nature photography. I need to purchase some software to assist me with post-processing my photos. Which one is better for landscape and nature photography. Will Photoshop or Lightroom do, or are there any other tools better suited to processing this type of photography?
Photoshop and Lightroom are photography tools, not Landscape photography tools or Portrait Photography tools, etc. Either will do fine for Landscape photography. Each tool has different strengths and weaknesses; many pros use both tools since they each excel in different areas.
To decide which tool you need requires you to determine what you want to do with your digital darkroom? Do you need digital asset management? Do you make basic corrections such as brightness, whitebalance, perhaps curves? Do you frequently make use of filters, find you need to dodge and burn or mask out objects or sections of images? Do you work with RAW or just JPEG? Do you frequently print or do you send your images out for reproduction? Are printer profiles and proofing important to you?
By the way, here are my answers if you asked these questions above: (L=Lightroom, PS=Photoshop): L,L,PS,L,L or PS, PS.
As mentioned by cmason, it isn't really about the type of photography -- but more your needs in post production.
Lightroom provides assistance with managing, retouching and sharing images. It is designed around a RAW workflow, I'm not sure how well it works with a JPEG workflow. Lightroom also has a non-destructive workflow, any edits inside of Lightroom can always be reversed or changed.
Photoshop knows nothing about managing and sharing (without the help of Bridge), but is very, very good at general image manipulation -- all the way from toning to building an image from scratch. The image format is irrelevant, it can open and work with formats you will probably never see in the wild. It allows you to do anything, even things that you probably shouldn't and it is easy to make destructive edits.
Because Lightroom is less expensive and assists with managing and sharing, I tend to suggest that photographers get it first. If they find themselves limited by Lightroom's retouching tools, Photoshop works very well with Lightroom.
My standard response is 'use them both and decide which one you like more', and both do have demos available you can try and then pick.
However, in this circumstance I'm actually going to say that for a landscape/nature photographer you might be best with Lightroom (please feel free to disagree with me in the comments). I've used both tools for about 10 months, and I use them both for very different reasons, however I've found that 95% of the time I work on a landscape photo all I'm doing is very minor adjusts to colour tones, contrast levels and the like rather than any heavy editing and that's where Lightroom shines.
The other side of the coin is as soon as you want to push past to more heavy editing you'll find Lightroom quite limited, which means dipping your toe into the waters of Photoshop. Both have their merits, but heavy editing is usually done less on landscapes (or at least, I should say on my landscapes), and Photoshop has gotten to a stage where the simple things aren't always as simple.
Increasingly you can do in lightroom what used to require photoshop. Lightroom is a lot less expensive than Photoshop. I doubt many photographers need more than Lightroom, or Lightroom and a few plug-ins. Or perhaps Lightroom and Photoshop ELEMENTs.
My recommendation: buy Lightroom. Don't consider buying Photoshop until you hit something that you can't do without it. When you hit that point, try the elements demo and see if it'll do it. If so, you still don't need Photoshop.
Remember, it's not "Lightroom" or "photoshop". Given the cost of Photoshop, it's "lightroom and these other toys" or "photoshop". And most photographers that have taken up Lightroom and worked at learning how to use it rarely seem to use photoshop much; most of the "photoshop centric" photographers I know are that way because they were doing all of this before Lightroom existed and that's where they're comfortable.
As far as I'm concernered, there's no reason to START with Photoshop any more. Start with Lightroom, and then decide when you hit needs that it can't do and figure out what tool will solve that need. Chances are, you'll never need photoshop to solve it.
Photoshop can do a lot that Lightroom cannot. Where Lightroom can make adjustments to your photo (e.g. adjusting contrast, sturation, brightness, etc), Photoshop can also do a lot of manipulation, e.g. removing, adding elements to photos, combining photos in new interesting ways. Quite a lot of the functionality that Photoshop provides is something that the average landscape photographer isn't concerned with, as they just want to portray reality as they saw it when they took the photo, and for this the adjustments that Lightroom supports is often sufficient. Also, nature and landscape photo competitions often forbid the use of photo manipulation, but adjustments are allowed.
So if your primary concern is Landscape, then Photoshop does probably not offer a lot of extra functionality that you would require. HDR is probably one thing that you could use. But you could get dedicated HDR software for a fraction of the price of Photoshop.
You could also try checking out Capture One, or (if you're on a Mac) Apple Aperture. Both are alternatives to Lightroom.