My point-by-point comparison:
Vocals: I love both singers. LaBrie was getting pretty weak live for awhile, but now he's as good as ever. I saw them a year ago and he was slightly off on one note during A Change of Seasons...completely flawless for the rest of the 3 hour show. Insidious Tangerine, I have no idea what you're talking about with him not being any good live, unless by "now" you meant 4 or 5 years ago. But yeah...I like LaBrie, although he does tend to strain himself just a little too much every now and then. I think he really shines in his middle range, where he sound comfortable, and his voice is amazing on slower songs. Russ is a great singer too, but I have yet to see him live. He seems much stronger with his upper range than LaBrie, and can also do more with his voice (rough metal vocals, soaring operatic stuff, quiet emotional singing, ect). Both singers are great to my ears, and they are both an integral part of their respective band's sound. Neither group would be nearly as good without the vocalist that they each have.
Tie.
Guitar: Romeo has the technical edge here, but Petrucci has versatility going for him. Victory is Mine...versatility =/= lots of weird techniques. Versatility is being able to play a variety of different styles convincingly and be an interesting player through it all. Petrucci has much more versatility than Romeo, who tends to stick to neoclassical stuff almost exclusively.
+1 for Dream Theater
Bass: Myung is great, but he seems to be somewhat limited in DT. I have his instructional video, where he does a fantastic bass solo at the end, and I really think that DT could benefit from giving him a little bit more room to work. I've also heard that he does a lot more in the Jelly Jam stuff, but I haven't heard any of it. But when he does get a chance to shine in DT, he really does shine. The SX guys get more of the spotlight though, and their basslines seem to play a bigger role in the music.
+1 for Symphony X
Drums: Portnoy's only weakness is his lack of dynamics in his playing...something which Rullo is also guilty of. I find Portnoy's drumming to be much more interesting than Rullo's, and he can lay down one hell of a groove when he wants to...6:00 for example. Both are very talented, but Portnoy appeals to me much much more.
+ 1 for Dream Theater
Keyboards: All the DT guys as well as Pinella are ranked in my top 5 keyboardists (along with Jens Johansson), and I really can't pick favorites here. They have all contributed great things to the bands that they are or have been a part of.
Tie.
Songwriting: Dream Theater have a wider variety of material to choose from, and they seem much more unpredictable to me. Symphony X have great songs, but after long periods of listening to them, the songwriting can start to sound very monotonous. DT are one of the very few groups that I can listen to for what seems like forever and not get tired or bored of their music. I do have to say that SX's orchestral stuff is fjooking awesome.
+1 for Dream Theater
Lyrics: Dream Theater wins. If you require an explanation for this, then you have my condolences.
+1 for Dream Theater
So DT wins for me, although I love both bands, and they are both among my absolute favorites. It's pretty clear that some of you are heavily biased towards one side or the other, which is a shame...I think that both groups have great things to offer.