I dont watch Miami so I cant comment on that. Right now I'm in LV end of season 7 and NY start of season 3, I know I'm far back at most of the plots, but here's my take anyway.
Acting-wise: I'd say LV, maybe because I dont see much chances for NY to express themselves much. Although there ARE scenes but I'm more impressed with LV.
Case-wise: LV before season 6 wins hands down (especially those serial killers). After season 6, it went downhill. Frankly speaking, I'm quite disappointed with NY's cases. There are many times where the case is so predictable and the suspect is just DUH! Or maybe it's just me (cause I watch like a gazillion movies a year).
Romance-wise: Right now, I havent seen much of Danny/Lindsay or Mac/Peyton yet so I cant comment that yet. I did become quite a fan of Smacked though. Their chemistry is just awesome! As for LV, I'm definately a Yobling fan, before season 6, there seems to be some moments and their chemistry is apparent but the discontinuity just piss me off. So, I'm leaning towards NY on this one.
Dialogue-wise: LV. Definately. Although some of NY's quotes can be quite funny but NY's lines seems a bit too cheesy for me. LV has a lot of witty remarks IMO. Just Brass' quotes are more than enough.
Others: One thing I might be quite annoyed about NY though, their roles are like interchangable. Hawkes turns from ME to CSI, that's fine with me. But every now and then he seems to be acting more like an ME. Then Flack's been acting too much like a CSI. And not to mention there are too many times the CSI team acts like police. I know technically they are, but still. I dont really remember them allowing others to put on the cuffs and do the chasing. Is it just me?
Of course, by all means this is just MY opinion. If you dont agree, feel free to comment. (with valid reasons though)