Truespace might be nice, thou i'd recommend trying an older free version 1st.
Amapi was my 1st real outside-bryce modeller, i found it quite easy to learn.

C4D should be a really good option also, specially considering the other tools it brings to the table, and the modular licensing system allows you to buy more functionalities later on if u want.

If you want to get into nurbs modelling, i suggest rhino.

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