I've work with many DSPs and here is just some of my personal experiences.
Use a floating point DSP if you really need to. In most cases, fixed
point DSP will work fine in most cases.
If you are working with high quality audio, which upto 24-bit, you
maybe out of luck with fixed point DSP. Although you can certainly
use Motorola DSP to do it, which is the few 24-bit DSPs, a 32-bit
floating point DSP will be easier and sometimes cheaper to work
with.
For Motorola DSP, I do hope they fixed this already. You can
only implement a circular buffer at 15-bit, eventhough the DSP is
24-bit. The modulo unit is 16-bit in size so if you have large
circular buffer requirement, (such as long audio delay), you are
better use TI.
For beginner, I recommend Analog Device. Their cheapest fixed
point 21XX are very good DSPs and the toolset is good too.
Motorola DSP toolset is rather poor compared to everybody else.
TI DSP is very powerful. They usually pack with tons of peripherals.
But they can be very distracting if you don't really need them. I
think TI DSP is for advance DSP user that needs a lot of extra
stuff and knows not just DSP but uC structure very well.
Anyway, good luck with your learning.
Gunship