I have done extensive work with the Framed SPI and DCI ports with the Si3000 codec.
IIRC Framed SPI only has a clock divider, whereas DCI has a period register which makes clock master mode more flexible. Not all codec setups require the controller to be in master mode, in fact IMHO master mode is generally undesirable. I seem to recall the other difference is the DCI can support multiple-word frames whereas Framed SPI is limited to a single 16-bit word.
The DCI can buffer up to 4 TX/RX words between interrupts, whereas SPI can only double-buffer. Since audio data is frequent transfers, this is helpful. However, the dsPIC 33F series then included a powerful DMA controller which makes the amount of buffering in the module irrelevant.
Not many people realized the Si3000 can be hooked up through a SPI port. I wrote a very well-coded, reusable driver to hook up as many as 3 Si3000 codecs to a single dsPIC33F by using the 2 SPI ports and the DCI port. The flow is all handled by the DMA controller (not present on the 30F series).
Always good to see someone else looking into dsPICs. It seems like some people have been turned off by the "dsPIC" name, assuming it is a special-purpose processor. In fact all its core (first off being a 16-bit core!) and peripheral features are really useful for general-purpose tasks. It's a remarkably powerful (while still cheap) series really.
33F has a lot of benefits over the 30F series too. DMA controller for one, another is 33F's faster and actually uses less power over the entire MIPs range. 30F will actually dissipate enough power to bake itself if you max it out and draw a lot from the pins without heatsinking. Downsides, 33F's only 3.3v (though hit has some 5v I/O compatability features) and has pretty low pin output drive current.