I'd first like to start off by saying that I'm HEAVILY biased in favor of Lita. I've never liked Trish and still don't. I will, however, try to stay as objective as possible when writing this.
I see this debate coming down to two things:
Wrestling/In-Ring Ability
Marketability
Wrestling/In-Ring Ability
In Lita's debut appearance on Sunday Night Heat, she blasted off a moonsault after Essa Rios had won the Light Heavyweight Title Belt. This completely shocked the wrestling world as, up until that point, there hadn't been much going on in the women's division (to my knowledge which, I'll admit, is limited). In her early feud with Chyna, it's my opinion that WWF underplayed her wrestling ability, as she was supposed to be seen as weak and "scared" of Chyna. This was kind of paradoxical in that she was hitting masterful moves on men and was scared of Chyna. Lita then, as we all know, transferred to be part of Team Xtreme with the Hardyz. Soon, Trish came on the scene as manager for T&A and the rivalry was born.
If we're talking about who revolutionized women's wrestling in the actual wrestling aspect, it's clear that it is Lita. As has been said by the Stratus fans previously, Trish took time to improve. The "two-trick" (Litacanrana and Moonsault) pony raised the bar that women had to meet. I actually think that Chyna should've have been the initiator, but there's just no way to market that dominant of a female among the rather pathetic women's division, which is why she was in a class with the men (the only woman to hold the Intercontinental Championship, etc...). Anyways, I believe that, because WWF/E (not sure which at the time) wanted to continue the Lita/Trish feud at a higher level, they mandated Trish's improvement from the measly in-ring ability to what it became.
I'll admit that Lita's injuries and on-screen roles have drastically affected her in-ring ability, for the worse. At the end of the feud (Trish's retirement match), Trish
was the better wrestler. However, Lita was the "in-ring master" from early on. In my opinion, this is what made the feud so enjoyable: In-ring skills vs. Deceptive tactics that would lead up to the two best female wrestlers on the roster (up until their "retirements").
Winner: Lita
Marketability
At Lita's debut, and throughout the beginning stages of her career, it was pretty clear (at least in my opinion) that WWF was planning to market her as a wrestler, and not just a pretty face. They had her 'face' role planned out.
When Trish arrived, the plan was to market her as a heel. She was stunningly beautiful (not the Lita isn't), which made it easier to throw in the sterotype that hot women are deceptive. She was marketed as having not much in-ring ability (whether she did or not) and as being one of the 'cheating' wrestlers that attacks people from behind and whatnot.
Lita was really pushed as a face and a crowd favorite during her ordeal with Stephanie, which culminated in her first Women's championship. That feud is overshadowed, however, in my opinion, by the Trish Stratus/Stephanie feud in which there was the "Trish is Vince's girlfriend" and "HHH shows Trish some moves" storylines, which led to the "divorce" of WWF/E's power couple, definitely a better marketing ploy.
Lita's injuries opened the door for a new 'face' and Trish was pushed by the WWE as kind of a bad girl gone good aspect. This is when she went through her in-ring makeover and skyrocketed in popularity. This is when Trish started most of her 6-championship journey (the 7th time was at the end, so it's not technically on the journey). When Lita returned, she did so as the 'face' she left as, actually 'saving' Trish from Molly and Gail, but another Lita injury took her out shortly after. When Lita returned the next time, Trish was so popular that, if Lita were to become a significant player at all, WWE had no choice, in my opinion, than to make her a 'heel' which they definitely outdid themselves in creating.
I'm not trying to say that Lita's injuries were the only reason that Trish became the star she is, but it did help. I'm pretty sure that Trish would have gotten the same type of push because she was more easily marketable in her storylines (Could you imagine if Lita had been the one involved in the HHH/Stephanie fiasco?).
Winner: Trish
Now, a comment was made to the effect of "That's why Trish will be in the Hall of Fame and Lita won't."
I'd just like to say that, despite my hatred for her, I agree; Trish will be in the Hall of Fame some day. She really became the face of women's wrestling in the past few years. I really think that they pushed her last title shot as the 7-time champion label has a good stigma attached to it. Will Lita be in the Hall of Fame? I believe she will be, but not as of now. I'm almost positive that we'll see another Lita return soon; I really don't think that they would want the future hall-of-famer to end on such a low note. When she comes back, they'll probably push her as a 'face' after the Mickie James inflammation is cleared up (can't stand her either).
In all honesty, I think that the women's division hasn't produced many Hall-of-Fame material wrestlers, so there's really no competition for it. In recent times (so I'm not going back to the Fabulous Moolah (though she's already a Hall-of-Famer isn't she?)) there's the following that strike me: Lita, Trish, Chyna. It's pretty sad, but as I sit here trying to think of others, none come to mind.
Just a quick wrap up. All-in-all, I'm not going to preach about who is better: Trish or Lita. My opinion, as was derived from my first sentence, is obviously Lita. As was explained above I think Lita beat out Trish in the actual wrestling category, as her early career was enough to best Trish's ending push. However, Trish's marketability is definitely on top. As was stated before, she sells.
Nice to see some actual discussion on wrestling on this board other than "Any one have any pics of Randy Orton's cock?!?!?!

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-margo