Well, I'm in a debating mood so I think I'll try to dispute each point. And no offense, these are also just my opinions.
1) He has much better fashion sense! One cant compare a snazzy red suit to a tattered old green blazer or an scary 80's jogger t-shirt.
One: what makes Mamoru's blazer "tattered"? It looks no more torn up than Seiya's. Two: it's his school uniform so he doesn't have a choice about wearing it. Three: "he" is actually a girl, so it's no wonder.
All that aside, I don't think Usagi would chose her lover based on his fashion sense.
2) He has talent! He is a fantastic singer, while Mamoru is.. um.. a fantastic gardener?
Mamoru didn't get into Moto Azabu and then Harvard by sitting on his rump.
3) Seiya has tons of girls falling over him. Mamoru has his future daughter falling all over him.
One, it's not Mamoru's fault that Chibiusa has a hopeless crush on him. (It's not entirely Chibiusa's fault either, but that's another argument...) And I don't see how having fangirls makes Seiya better for Usagi. In fact, they say celebrity romances never last. Now, this might be a valid argument for "Seiya is cooler than Mamoru" but not for "Seiya is better for Usagi than Mamoru is."
4) Seiya acknowledges Haruka's strength, while Mamory stupidly stepped in on one of her fights.
What does this have to do with Usagi? Again, this might be an argument for why Seiya is better, but does nothing to prove that she's better for Usagi.
5) Even as a girl, Seiya looks more masculine than Mamoru.
Uh, I'm not sure that's a good thing...
6) He makes his moola as an ultra famous idol, making movies on the side, while Mamoru gets along by being a busboy for a hotel.
Consider their respective backgrounds, though. Seiya comes blasting in from another planet and is an immediate hit on the idol scene. Mamoru, meanwhile, has to deal with the death of his parents and the loss of his memory, plus the fact that he's having dreams about this mysterious princess and the four guys who were supposed to be his protectors have been brainwashed and are trying to kill him. In spite of all that, Mamoru manages to land a steady job and pay his own way through college. I think raising yourself from the age of 8 is an equally, if not more impressive achievement as becoming a rock star.
7) He's unpredictable! As starfighter he (she) actually FIGHTS, he doesnt just show up and throw out some Hallmarky greeting along with a complimentary weed.
That weed has saved Sailor Moon's butt on more than one occasion. Yes, I will admit that the speeches are a bit... whimsical, but Tux does what he can in battles in spite of the fact that he isn't gifted with awesome powers like "Star Serious Laser." Mamoru has died for Usagi. Seiya has never done that, not even for her own princess. But all of this is a moot point, because being a good fighter does not make someone a good lover.
8) He's better looking and closer to Usagi's age.
True, he is closer to Usagi's age, but better looking? First of all, that's a subjective call, and second, they don't look all that different. Usagi even admits that she found Seiya attractive because Seiya reminds her of Mamoru.
9) He had a balcony scene with Usagi! Top THAT, Mr. Unromantic Mamoru!
Mamoru has kissed Usagi, and she has kissed him back. Mamoru has given Usagi an engagement ring. Mamoru has died in Usagi's arms after taking a blow meant for her. Mamoru's love for Usagi has given her new powers, new strength, and new confidence. Mamoru's kiss even brought her back to life in the R movie. Seiya has done none of these things.
10) When's the last time you saw MAMORU in Usagi's bedroom, and then using her shower?!
That's cause she's always at his place ^_~ At least in the manga she is. She even spends the night there a couple of times.
All that aside, I don't think Usagi would chose her lover based on his fashion sense. 
