Are male Rottweilers more aggressive than females?
Which is more aggressive male or female rottweilers?
To set the record straight... Rottweilers are not aggressive dogs in general! Please see my Are Rottweilers Dangerous? page to find out what the REAL Rottweiler is all about.
If you're asking 'which sex is more protective', then it's a different question, but there's no simple answer. So much depends on the individual personality of the dog, and although males are generally bigger and stronger they're not necessarily more protective. Females can be very good at 'guarding' their families if the need arises.
Please don't consider getting a Rottie because you think they are an aggressive breed, or any such thing. These are incredible dogs - loyal, loving, intelligent and very sensitive. They need an owner who really understands the breed and their needs. If you want to add a Rottweiler to your family I'd recommend reading a lot of the information on this site so that you know exactly how to raise him/her properly.
Best of luck.