Schumacher and Senna where both extremely good. But, if you go through the records and do a mildly fair comparison, Schumacher comes out on top.
Races/Average Points, etc, etc, etc.
You can not say that Schumacher always had the best car. The F2001 and F2002 being the exceptions, the other years the Ferrari was not the dominant car. Look at the results of his team mates.
Senna on the other hand, had it (in my opinion) alot easier with the cars given him. Again, look at his teams mates results.
If you really really start getting into F1, try and find some videos, doucmentaries about Schumacher and why most people involved in F1 conceed that there is Schumacher, a gap to Senna, Brabham, Fangio, Hill, Stewart, Clark, Villienueve (G) and then a very big gap to the rest....
Try and find a you tube video of Schumacher at Silverstone testing with Johnny Herbet, they explain one of traits that Schumacher uses that 99% of the others don't. The bloke is the best we'll ever see - love him or hate him.
On the rain thing, Senna was good, but you only have to see the reaction of the paddock, commentators when it rained and Schumacher was around.... Schumi - RainMeister.
His Dad used too make him go to the local kart track at 8 years old when it rained, made him practice in the rain all the time.