Just saw the weigh-in and I think Saul Alvarez handled himself very well. He was calm, cool, and collected, and refused to accept a belt when Floyd handed it to him, making Floyd look a bit stupid on stage. That being said, weigh-ins don't really mean anything and when it comes time to fight, only their skills will matter and whatever happened at the weigh-in means nothing, more or less.
I've seen several of Alvarez's fights and what I have noticed is that he is a good puncher and moreover he has good defensive abilities. He doesn't take unnecessary punches, seems to block well, and has the movement to make his opponent miss. One thing I've noticed in his past fight with Austin Trout, though, is that his conditioning could be improved and he seems to fight in spurts and at times lacks energy, similar to Arthur Abraham.
Mayweather has made a career out of cherry-picking his opponents, and personally, I don't find his list of opponents as all that impressive (as many of his biggest opponents were old by the time he faced them). That being said, Mayweather is undeniably a great fighter, is very fast, defensive, and also has the power to hurt guys. It remains to be seen how his age will affect his fighting. He is getting older and one of these days I'm expecting his age to catch up with him. Some fighters fight well into their late 30`s and even 40`s but usually fighters who rely on speed (like Mayweather and Jones Jr) tend to burn out faster. When their speed goes, it`s over for them, because that is 90% of their fight. Speed goes first so these fighters tend to get older quicker than others (this is also why fighters in lower weights tend to age quicker).
It will be interesting to see how the weight dynamic affects this fight. Both guys were very ripped at the weigh in. Some people have said Alvarez looks "weight drained" and that may be true, but the other side of the coin is that Alvarez is much bigger than Mayweather. Hopefully Canelo can use that to his advantage. There is a reason that weightclasses exist and it's because bigger guys are stronger and will usually win in a fight (see, heavyweight Tyson Fury KO cruiserweight Steve Cunningham a perfect example of the bigger guy using his size expertly to KO the smaller guy). If Alvarez could enter the fight weighing 170+, I think that would work in his favour.
I predict the fight to go the distance and I think it will be competitive. Robert Guerrero was able to win 2 or 3 rounds on Floyd, and I`m pretty sure Alvarez is better than Guerrero, so I could see Alvarez winning more than that. I won`t pick a winner other than to say I think it will go the distance and a relatively close fight (say, 116-112 either guy, 115-113 either guy, or even a draw 114-114).
Not a fight I would bet on personally.