FW was totally new to me, so I did a lot of research on that before I made my order. I only saw Thunderbolt in passing, and did not research that one yet. These units are designed to be expanded and function fine on FW, so Thunderbolt may be overkill (for now anyway).
The mic pre-amps are wonderful on the 24. I actually bought my first-ever condenser mic at the same time, and learning to use that properly was an interesting experience unto itself. I found a neat trick that can only work well with good preamps... my voice very strong overtones that the condenser mic picks up "too well," so if I record a condenser and dynamic mic in tandem, I started using R-MIX SONAR to low pass the root notes, and doing the same on the dynamic mic to high pass the overtones. This combination made for a very smooth sound that kept the high end without the "sting" that the condenser mic was giving it.
One thing that is sort of "tucked away" in the Mix Control manual, that is important.... When you go into "Input FX->Router," the "Loop Back 1/Loop Back 2" are what is being sent to SONAR. When "Off" they are sending the mix, so if want just the FX they need to be set to FX(Anlg 1/2), and if you want the dry signal they need to be set to Anlg In 1/2.
It is easy to spend a lot of time on just the Mix Control panel initially, so once you have mixes set up to your taste, be sure to save your settings.