Since Chapter 280's release some few hours ago, I've been going through the latest blog posts (the comments especially), and arrived at the conclusion that a lot of you aren't very appreciative of the possible Lucy and Erza no. 2s, that the latest update seems to imply. I don't like the general gist of it either (I'm still a little stingy on Lucy's battle with Angel, and I just don't like Karen at all), but I don't think it's such bad thing altogether.
What I'm trying to say is that I don't think Yukino's a Lucy no. 2 at all--and this is mostly just because they share the same magic. I'd like to see Yukino go against Lucy, and I'll bet the fight will be nothing like an Angel-vs-Lucy battle (if it hadn't been for Hibiki, that fight would have been tragically one-sided). Yukino has Ophiuchus, Lucy has Loki. Thanks to Ultear, Lucy has powered-up significantly, and honestly, we don't know the full extent of Yukino's abilities either (too bad Hiro-sensei limits the fights to a chapter each--imagine if he went ala-Tite Kubo and extended battles to over 25 chapters! XD). I don't know why some people don't want to see them go against each other, because I think it's something to look forward to.
As I recall, the strength of a Spirit is relative to the strength of his/her contractor. Ophiuchus is theCelestial Spirit after all, so I think Yukino summoning him was a (if not the) key to her defeat--it must have took a vast amount of magic to call him forth, and that took it's toll on the girl. In that sense, she still lacks control over her spirits--much like how Lucy used to be. Yukino also seems comfortable with summoning several spirits at once (or consecutively), so imagine how it would be if she has more than two golden keys at her disposal (I'm not entirely against the idea of some of Lucy's keys--not just gold ones--falling into the hands of Yukino; I kind of think it'd be nice too).
My point here is, overall, I think that, against each other, both CS Mages will put up quite the fight. I don't think Yukino's weaker than Lucy at all, neither would her power-up give Lucy much of an advantage against a legendary Celestial Spirit that may possibly go berserk if not handled properly (what is this, an object? XD). If you ask me, a fight between them would definitely be epic.
(I just wanted to get this out. XD I was a little shy about joining your conversations, but I wanted to share my take on this issue too. I hope you guys can provide feedbacks!)