> How many semi-spaces do you think of having in this generation? Most systems have and recommend two.

I suppose we would need to experiment, but for what I have seen I think two or three :) What do you think?

> If it is a worse tradeoff to managing steps then I would suggest to stick to the steps.

I think any memory increase that scales with the number of objects will be a big concern.

>  we need to add a step and manage it which is more complex than for example when tracking ages, incase of a change we only change the age threshold which looks simpler to implement.

Exactly, but if with "tracking ages" you mean store per-object ages then the previous concern applies :( Adding sub-generation steps should not be much more complicated IMHO.