Gah! Just what we need! Another damn greenskin!
Just kidding
As the resident Eldar player I feel obliged to be disparaging about orks. 'orrible smelly uncouth things that they are
I'd say dont rush into choosing an army. Take a look at how the game plays in person, ask folks for opinions, browse the GW site and ponder the models, and generally take your time with it. Something to remember is that you will probably be spending far more time painting the models than playing with them, so be sure to pick an army that appeals to you visually.
From a painting perspective, my comments on the 3 armies you picked out would be:
space marines: nice and simple, but can get monotonous, as they are all largely the same colour. You'll learn a lot about choosing good complentary colours and working with a limited palete, though. Also, weathering vehicles is fun, and SM tanks are some of the funnest and easiest models to paint well (I think)
orks: big, chunky models with lots of personality in the faces. They're very customisable, so you can swap arms and legs and li'l accessories around to make an army of pretty much unique models. Somewhat fiddly to paint, as they tend to be covered with lots of bits and bobs, but their large size makes highlighting and shading quite easy. Also, you can have a lot of fun weathering their vehicles up the wazzoo. Some very simple techniques will produce some really nice looking ork vehicles. Colour-wise, be prepared to paint a lot of greens and reds (with a bit of thought, though, you can have quite a variety of colour in an ork army.)
eldar: hum. As someone who has played eldar for about 20 years, I have to say that they're the prettiest army out there (but I am well biased on this one
) Each of their units is a different colour, so you get a lot of variety in your painting. (OK, the guardians and vehicles use the same palette, but you can choose it yourself. If you want pink vehicles with purple stripes, dat's ok. Hideous, but ok
)They tend to be very clean-cut designs, though, so they are rather unforgiving of sloppy painting. Their vehicles tend not to look so good weathered (sleek and sci-fi is the order of the day) so if you fancy grim and gritty perhaps you should look elsewhere. Smooth blends, bright colours (generally, but not exclusively) and neat painting is where it's at for eldar.
With regards to play-style, all of the above armies can be played any way you like, really, but they lend themselves well to:
SM - being tough and hard to kill. Steamrollering the opposition. Pretty flexible units (e.g. can kill stuff in CC, shoot stuff to death, and even take a pop at a tank with the same unit.)
Orks - you can go the mekboy route and have lots of ramshackle vehicles and bikes n stuff, or go the green horde and just run hundreds and hundreds of boyz at your opponent. The latter option is actually a very hard tactic to beat, but damn that's a lot of boyz to paint
eldar - a glass hammer of an army. It helps a lot to know each of your and your opponent's unit's strengths and weaknesses. Kinda the opposite of Space Marines. With Eldar, if you run e.g. Howling Banshees at a unit of marines, they'll kill em in short order. If you run the same banshees at a unit of orks, the orks are likely to win. Choosing targets carefully is important, as well as using your own units to boost other of your units' abilities (e.g. the Eldar Farseer HQ unit has a lot of buff/debuff spells) If you screw up with eldar, you'll get hosed, as they are rather easy to kill. If you do it right, though, you'll hose most opposition. Eldar are kinda fiddly, but I love em