You have 19 units to choose from not counting HQ!! Marines do have maore units, but most of them are variations of one thing, like a LS or their tanks!
Just for the sake of argument, go back and re-count those. As you're not going to count SM units that are variations on a theme, don't count the Jetbikes seperate and don't count the Waveserpent/Falcon/Fireprism as seperate. Also don't count the Storm Guardians seperatly from the Guardian Defenders, as they're variation of one thing. Dire Avengers could be completly discounted. Banshees and Scorpiuons could be counted the same (both HtH aspects after all, just banshees concentrating on anti-armor while scorpions concentrate on mass numbers of attacks). Farseers and Warlocks are pretty much the same thing (psychers you add to other units or take together as a HQ).
You get the idea, I won't continue further. Besides, one person's idea of a variation on a theme is another person's idea of a completly different unit. That's a bit too subective. Not even counting the number of ways that different squads can be geared-up to fill vastly different roles on the field.
Hardly a valid argument, is it? So "variations" or not, count the ACTUAL number of units.
The origional point stands, SM have a wider variety of units available. Some of which are just as hated by Eldar players as Wraithlords are by SM players (*cough, cough, WHIRLWIND, cough*). The SM community just gets MORE stuff. Period.
Some people think that's the reason new people pick the SMs. Others think it's a problem with game balance. I think it's a combination of the two. WHFB is a good example of the same environment with balance. There's no one dominant army. Sure, people get Army-of-the-Month with the new ones and all that, but there's no one army that has anything close to a 50% market share. Two games, by the same company, very very similar in both armies and background. One has a 50% phenominon and one does not.
Sounds like there's something wrong there to me. But hey, besides trying for a position with GW as a games developer there's not a lot anybody's gonna do about it.
Well, maybe a "Balance the Universe!" campaign on the GW Games Dev board, but you'd have to get them to admit that there's a problem first.