There has been much debate on who would be a better boyfriend for Tohru. Kyo or Yuki Sohma? Well, here's what my opinion is.

Yuki Sohma - Yuki on the outside seems like a very nice, soft-spoken guy. Although this is how he appears on the outside, I feel he is a bit different on the inside. Yuki can sometimes come across as intimidating. Well, to me that is. Especially around Kyo. There is also the fact that he was abused by Akito and felt almost abandoned by his older brother. So Yuki seems to be hurt and angry inside, but covers it up with a "sweet guy" demeanor.
At a certain part in the manga (And perhaps the anime too, I don't remember) Yuki mentions something along the line of "I'm not a really sweet nice guy like most people think, I just act that way so people will like me" or something like that. Those arn't the exact words, but what he said was something like that. I'm not saying that Yuki is really a mean guy, because I do feel, at heart, he is a real nice guy. For example, Yuki is always very polite and kind to Tohru.
Hanging out with Yuki would probably just be nothing but awkward silence (For me! Not necessarily Tohru). There are times, like I said earlier, where he's almost a bit intimidating. Although, I'm not saying he's always like that. Yuki has a very sweet and benevolent personality, so as long as your not Kyo or Ayame, you should be on his good side. Even though, I said we'd have nothing but awkward silence, I admit he seems like someone that you feel more comfortable around once you got to know him better.

Kyo Sohma - Kyo. Kyo on the outside can come across as a hot-headed individual, but on the inside, he is something much sweeter. Kyo to me seems like a guy I could hang out with. He seems like a really sweet guy, that just covers it up with his "Bad Boy" attitude. Yuki can be really mean to Kyo sometimes, and I always feel bad for him. lol Even though Kyo HAS started some of the fights between him and Yuki, there have been times where he's just trying to enjoy himself, Yuki comes and ruins it by irritating him.
Kyo, as you know, is the year of the cat. For as long as the Sohma family had this curse, the once cursed with the cat's spirit was always seen as an outcast. So Kyo too has had his share of hard times. Akito hates him and finds him vile and disgusting. His father despises his very being and above all this, Yuki hates him with his life. In the manga, once the person cursed with the cat's vengeful spirit turns 18 or so, (Sorry, I can't remember the exact age) they are to be lock in the Sohma house for the rest of their life. Never again allowed to go outside. Kazuma, Kyo's teacher and father-like figure (Awww) says that as long as he's alive, he will not allow that to happen to Kyo (Awww x2). Because Kazuma see's that Kyo is changing. He's able to rise above all his hurt and finally moving forward in life. Of course, I say this is Tohru's doing. (Hurray for Tohru!)
I've also noticed that in both the anime and manga, whenever Kyo and Tohru are hanging out alone, Kyo comes across as very sweet. (Until Yuki comes and ruins it! lol) Kyo seems to be more outgoing than Yuki. Kyo likes to bug/play around with Tohru, where as Yuki usually doesn't. The one BIG problem about Kyo, is that he has Kagura on his tail. Would you want to be on the receiving end of an angry Kagura?!?!

Conclusion: So who would be the better one for Tohru?
Yuki? Kyo? Well it's hard to choose because to me, neither would be a bad choice. But if I had to say to choose, I would say...Yuki. Yuki and Tohru just seem to go together like eggs and bacon. lol They are both nice and polite people. And you can see that Yuki really cares about Tohru. Of course that's not to say that Kyo dosn't have feelings for her either. But I don't know, even though I think Kyo and Tohru would be cute, Yuki just seems like more of a Tohru guy.
As for me...Who would I pick of the two? Kyo. He's seems like a really nice guy who'd be easy to get along with. (Except there's a slight problem with Kagura...) Plus he's so cute~! (Well, Yuki is too, but Ayame is hotter) But Kyo seems to be a bit more outgoing and more fun. But in actuality, I wouldn't pick between Yuki or Kyo, I'd pick Hatsuharu! Woo-hoo!