First off, great points but I gotta counter-point...
Just to play devil's advocate, I'd say that Lesnar would actually be a better acquisition. Here's why I think so.
1) A lot of young fans, to whom the product is aimed at these days, do not know how much of a legend Sting is. Now that is partially because of their youth and partially because of the reason that the WWE have never exactly promoted Sting because obviously he never worked for them. They have a better chance of knowing who Brock Lesnar is given the popularity of MMA.
I'm from Norn Iron so I gotta ask - kids are allowed to watch UFC? Even now WWe has gone PG, their DVD's are '15' or (more often) '18' rated by the BBFC (meaning you have to be at least 15 or 18 to view). This is due to the violence - if we where to allow small kids to watch the ultra violence of UFC, we'd be strung up. WWe might be PG now but PPV is paid for by adults and we know who Sting is and remember the way Brock walked out on us before.
2) Tied into my first point is the fact that Sting has almost no mainstream recognition. WrestleMania is the biggest show that any wrestling company can put up at this moment and logically it will draw in as many wrestling fans as it can with or without Sting. Brock can help the WWE draw the people who are not interested in wrestling into WrestleMania.
True... buuuuutttttt... at the height of the Monday night wars ratings between the two companies was cresting 10, now unopposed Raw doesn't hit 4. Brock might bring the mainstream to watch one event. Sting could bring back wrestling FANS and bolster the stagnant figures. One off versus long term?
3) Brock will put on a better show as he is more or less in his physical prime unlike Sting. Sting might mean a lot more to wrestling fans than Brock Lesnar but by putting on a bad show he stands a chance to disappoint the fans that are clamoring for him to come to the WWE. I'm sure Sting does not want his only match in the WWE to be a bad one.
I have to disagree, Sting might have lost a step but he has been wrestling these last number of years, Brock has not. Brock is just as likely to put on a bad show (WM XX amigo?). Along with having been out of wrestling this length of time Brock is MMA now and as such would be expected to wrestle that style. With the exception of Angle/ Joe at Lockdown name another successful MMA edged bout? Shamrock, Blackman, Abbott, Lashley, Kozlov - none of these guys have really been able to translate MMA to sports entertainment.
4) If Sting never signs with the WWE it will be better for his legacy. It's probably a difficult point to understand but here's why I think this way. People talk all the time about how Austin vs Hogan would have been the greatest match ever. Now imagine if that match had happened. Do you think that the people would talk so much about it? NO. They would probably remember it as the Hogan vs Rock match at WrestleMania 18: Epic atmosphere but not great from a technical standpoint.
In the same manner if Sting never signs for the WWE, his legacy would assume mythical proportions. He would be remembered as the greatest superstar never to have worked for the WWE and a one of the greatest of all time inspite of that fact. I think that him working for the WWE will probably spoil that aura about him a bit. Sometimes it's better to let the people salivate at the possibilities rather than actually go through with it.
Austin versus Hogan - greatest match ever? Love both these guys but greatest match ever? This match screams not living up to the hype. Hogan versus the Rock - the great one just got decimated by the immortal one, this one match re-energised Hogan in that the fans illustrated that they (for once) would not be forced to cheer Dwayne over their beloved Hulkamania. Sting deserves his place in the most recognised HoF and if he is going to be in WWe's HoF then he deserves to have a WWe segment of his career (no matter how brief), otherwise his place just feels false.