If you're in the design phase, have enough time and have a good background on power electronics design, I would suggest going for discrete design and implementation. This will give you enough flexibility to create an optimum design.

Otherwise, you've to do a survey of controller components available in the market, and do the design taking the manufacturer's guidelines also into account. This may take a lesser time.

Also, a lot depends on if it is going to be a product or for a specific use only.

The use of a microcontroller can drastically reduce the parts count and cost of your design. Through a sine look-up table you can generate the needed PWM signals can can then be use to drive your power components,whether tyristors or power MOSFETs. If you can be more specific may be I can be of further assistance.
Good luck !

Have some experiences with invertors, specifically 50kVA static invertors for seam welder application (aerosol cans and stuff).
Dedicated 3-phase PWM ic's are available from Siemens and
GEC (MA828), these interface readily with any cpu or microcontroller with minimal software overhead. Also IGBT stacks are available from Semikron which have built-in drivers and firing control circuitry.
Certainly, you could do all of this yourself; but unless you intend selling several hundred units, the development time versus performance enhancement is hard to justify. Also, chances are that you would not significantly improve on the performance of the "dedicated" option. You could implement your own PWM generator quite easily for instance, but at 20kHz this requires significant cpu time - even for an 80552.
My suggestion therefore, go the dedicated route - there'll be enough other problems to keep you occupied.

Hope this helps
Russell Fowler
Cape Town, South Africa