How to determine mutations

While fiddling with mutated humans and altered animals I have been thinking about the topic of how to generate actual mutations. Various approaches seem viable but I currently am highly divided about which one(s) to support in Gaia Gamma.
Currently I see the following variants:

• Roll for a fixed number of mutations (e.g. maybe 7 in total, 1d6 of them physical, the other mental).
• Roll for a completely fixed number of mutations (e.g. 2 physical, 4 mental or vice versa).
• Rate each mutations according to its power and roll for mutations until you reach a certain power limit (e.g. 10 for mutated humans, 6 for altered animals).
• Select mutations instead of rolling.
• Select one or two mutations, roll for the rest.

And then there is the topic of deficiencies: Should they be included in the mutation lists or should they be gathered on an extra list? So that you can decide to roll for deficiencies if you want to have more or other mutations?

What do you think - should I support all these variants or just one? Which one? Which do you like best? Which did you prefer from earlier editions of Gamma Games?