I've played the Guard since the mid 90's, I've had Marine armies and a Necron one. My main oponents throughout that time have been Orks, Eldar and Marines in that order.
In 40K races are loosely grouped into "types". The Guard and Orks are what are termed "horde" races. That is their primary tactic is lots of infantry rushing at the enemy. Or in the case of the Guard standing and shooting them and dieing, in vast numbers and still having enough to win.
So with either race you are choosing an army that will require a lot of models. If you like that part of the hobby they are definitely good choices.
The Orks like to get into Close combat the Guard like to shoot the hell out of the enemy thats the main difference from the game play point of view. Neither race IMHO are forgiving to new players as both require good knowledge of the game. However you will find everything you need to know tactic wise here:
http://www.40konline.com so take a look there and have a think. There are lost of mini reports of battles between the various races in the forums and stuff. Enjoy.
Personnally the Guard are awesome. You can go all tanky, shooty, Red Army infantry slaughter, specialist air, specialist cav, specialist demo etc they are just so versitle. But they do die. I probably loose more games than I win at the moment but everytime I loose I anihilate the enemy to the point its basically a draw. That to me is what makes the guard. The slaughter in the name of the EMPERAH! Oh and Commisars are also very awesome super sweet.
If you want an easy "noob" friendly army your best bet is Space Marines but everyone plays Marines and they are kinda a bit ghey. Also playing a slightly more complex army from the outset may in the long run lead you to apreciate the game more and be better at it. The more you loose the more you learn for next time with 40K IMHO.