i find that both (for me at least) are comparable in speed. and i text a whole helluva lot (800-900msgs per month). however both methods have their advantages over the other.
qwerty: i find that i abuse the keyboard when texting with qwerty. what i mean is, that i use punctuation and accents. a lot of my friends ask me, "Why do you use commas, colons, semi colons, hyphens etc... in text messages" I respond, cuz i have them readily at my disposal (and i am a grammar freak). additionally, you can easily type numbers and special characters such as the ampersand (&). (though it can be argued that you can type numbers easily in predictive text too by just holding down the key of the number you want.)
predictive: the biggest advantage clearly is one handed typing. this is why not having a keyboard is not a big deal to me. i can text people quickly and watch 24 at the same time. Its very easy to text without looking with predictive text. You may have to look down occassionally to choose the right word, but still one handed operation rocks. Additionally, not having a keyboard means the phone will be much smaller (in most cases) so you dont have to have a huge phone to carry around (though if nokia wants to send me an E90, i wont complain)
You see kids, each method of text input has advantages and disadvantages. Experiment with each so you may find a method comfortable for you. Different strokes for different folks.