Is there a great difference between the two manufacturers- Canon and Nikon?
They are very, very different companies. Canon dwarfs Nikon in size. They both sell cameras, lenses, and optical tools like scanners and binoculars. Canon also has a major business equipment business selling copiers, printers, etc. Nikon is much more focused on photography equipment and SLRs in particular. Canon makes their own sensors. Nikon buys theirs from Sony.
I'm not sure that any of that is relevant. Both were in the DSLR market from vitually the beginning of that market. Both now have healthy balance sheets. Canon has a larger share of the market, but not by a huge margin.
What about the range of equipment, do both offer similar diverse amounts of lens etc or does one struggle against the other?
Both sell similarly broad lines of gear. Both sell entry level, mid level, and professional level DSLRs. Both are widely used by professionals throughout most areas of the industry. Each has pockets of dominance (Canon with sports photographers, Nikon with landscape photographers).
The complaints that I hear from Nikon shooters are that they are slower in bringing out new technology (IS, FF) and that their high end lenses cost more. This is particularly true in the very long (400mm to 600mm lenses). The complaint that I hear from Canon shooters is that specialized third party accessories sometimes come out for Nikon first or Nikon only. Also, the used equipment market for very old lenses favors Nikon.
Is the dust reduction machine (Canon 400d) highly benefical or is it not an essential?
While dust reduction systems sound like a great idea, I've not heard good things about them in practice. It's not clear whether the complaint is that they don't help at all or that they help but don't solve the problem. Either way, you will still have to deal with dust issues. I don't think I've seen anything that indicates that the dust reduction systems should be a major selection factor.
In my opinion, the differences between the four most similar brands (Canon, Nikon, Sony, and Pentax) are relatively small at the entry level. They start to widen only as you get into more exotic gear. Even there, the gaps have been narrowing, at least between Canon and Nikon. If you don't expect to buy very specialized stuff, I'd pick more on the basis of which camera system you are the most comfortable with. If you do plan to get specialized, talk to the specialists in those areas.
I left Olympus off of my prior list because their DSLRs are different. They use a smaller sensor, which has benefits (longer reach) and drawbacks (more noise). Their sensor is also a different aspect ratio, which adds it's own benefits and drawbacks. They are also much smaller, which mitigates many people's biggest complaint about DSLRs - they are too bulky. I'm not trying to discourage someone from buying an Olympus. Instead, I think they should be viewed as a significantly different alternative.
One other thing I like to stress is that if you have friends that are into photography, consider buying into the same brand that they have. That gives you more opportunities to share equipment and knowledge.
Each brand has its strengths and weaknesses. Find the one that is best suited for your particular needs.