Theyre not quite the same, space marines are the ultimate warriors for the humans empire genetically made and enhanced occasionally agumented with technological implants and have the best of the best in terms of equipment (power armour) tanks, training spaceships etc
problem is there are only a few of them relatively speaking in small chapters.
Imperial Guard are more comparable to the troops of today (how they look etc) theyre no were near as effective as space marines in the warhammer 40k universe, they rely on sheer numbers of poorly equipped everyday soldiers to hault their enemies.So the similarity is, that theyre both fighting on the same side, but ones vastly better at it than the other.
I was abit dissapointed that the fifth 'race' is Imperial Guards, afterall you could control some of their units in the single player campaign, necrons would have been interesting (think terminators) or maybe even Tyranids or dark eldar etc, but then i saw the screenshot of the IG tank and now i cant wait , im sure their will be more tweaks to the existing gameplay as well as the additionally fully playable IG.
~Cent~