I can certainly recommend pet insurance - if your dog is sick or injured it is a lot easier knowing you don't have to worry about whether or not you can afford to have them tended to.
In regards to the different insurance, there are a lot around which are like the RSPCA plan. With these, you are covered for a proportion of all vet expenses (generally 70-80% depending on which plan you pick) up to a certain limit. These generally won't cover routine care & vaccinations (some can cover it, but the extra you pay is pretty much the same as the benefits, so I don't think it's worth it). The advantage of this type of cover is that smaller cost visits, such as grass seed removal, will still give you a benefit - but if you have expensive treatment, you can still be out of pocket quite a bit. A $3000 vet bill would leave you $600 out of pocket on the 80% plans.
The other plan type (ie. petplan) covers you for 100% of costs, but has a $100 excess. So a cheap vet visit you will get nothing much back, but you won't be more than $100 out of pocket no matter how much the treatment costs. I think you can get cheaper plans with a higher excess too.
Other things to consider are whether illness as well as accident are covered, and whether there are extras included and to what level. PetPlan has cover which can cover a lot of things such as rewards for lost dogs and the purchase price of your dog if it is stolen or dies (not much benefit for my 3rd hand mutt from RSPCA, but great benefit for my little pedigree puffball!)
The other major consideration is older dogs. Most policies will not cover a dog for illness if they are signed up after 9 years, but will cover them if the insurance started before then & is continued. They will still cover older dogs under accident only policies. PetPlan will continue to cover older dogs, but the benefit level is reduced.
The conclusions I came to for my dogs was that RSPCA insurance was the best one for my older dog, but for my younger dog PetPlan is best. However, when she reaches 8yo I will change her over to RSPCA as well.
Most of the policies around are similar to RSPCA, and I think are underwritten by the same insurance company. PetPlan is the only one I have found with an excess, which I like because I am more concerned about a big bill than little ones. In general you get what you pay for - the better policies are more expensive.
Hope this helps.