i think alot of people are asking the same. not sure where we are in terms of the numbers in each at the moment but i would say the class A will be a bit more competitive mainly because there will be a bit more money spent on the bikes. class B will still have plenty of really good riders in it. most of whome can whip my arse. but i'll be in class A getting it whipped there. i think most novices coming into racing will opt for class B
For the cost off Tim's bike you could get a 583 prep it and race it for the whole year including all fuel, one front tyre and two rears. So if its cheap you are looking for then 583 could be the way to go. I say could be as blow-ups can be costly and if you intend racing a 6 to 8 year old 583cc I would say that risk is greater than if you go for a 620.
Other than mechanical failures and crash damage the running costs for the year should be roughly the same in each class, so that just leaves the initial cost of a bike and with people like Paynep selling race ready 620s for a very reasonable price then even that initial cost isn't that far apart.
I'm sure both classes will have a wide range of riding talent, there may be a few more novices in the 583 but the fast guys will still be fast just like the fast ones in 620 and they will set the bench mark that will allow you to judge your progress.
As they say you pays your money and makes your choice