Both mpc and reason runn a resolution of 96 ppq so timing isn't a issue, processing speed is different because the pc runns through kernel multitasking which is kinda shabby opposed to MAC osX which runns mulitple kernel drivers in multi tasking. the mpc processing is not multitasking and therefor trustworthy of crashing. It's also portable, but so is a laptop, but the mpc oppposed to a laptop is far more flexible when considering working with mouse only which is not flexible.
Now as far as we can speak of quality, they both relate to the environment they reside in. The mpc and pc isn't much to talk about, in this case a pc will alternativly take over outboard funtions which are emulated from the real thing. So far as compressing goes, I have not yet found a vst/dx2 that was capable of doing the same thing my hardware compressors can do. Especialy on the comp/lim/gate combo it fails to dither, truncate a result similar to a harware compressor's signal processing. Anything close to usefull in this case was T-racks and PSP Vintage Warmer ( both tube emulation ).
For reason itself as a production platform, I would recommend a multi I/O dsp cards. Reason enables like 48 output directly ( without rewire ) so if having a good analogue console you can mix and master your works through it. This is much better than using the Rewire utility in conjunction with SX only ( no multi I/O, no Console ) since the blending is analogue and not calculated/emulated/dithered/truncated. But that's my opinion when talking about quality, the funtionality is cool since with Rewire all outputs from Reason go into the bus mixer from SX which enables the usage of vst's/vsti's and the sx score. The downpart is that Reason itself as a platform doesn't include a midi out, which means that you can control Reason from your MPC but you cannot use Reason to control your other synths and midi gear which mean in my case the program goes out the window since I don't really like the sampling and emulating synths. The hardware equivalent sounds much better and the filters in Fruityloops tend to sound better ( read phatter ). What left me was the drumming section, so the rex fileplayer and the Redrum which I used a lot together with Recycle. I named it a Random break generator since both combined enabled me to make breaks at random very quickly, I dithered them ( exported ) dry ( no fx ) and loaded them into my sampler. I found that was the only function of reason that I found usefull in my setup. When your setup consists of multiple gear with midi it would come very apparent, very quickly that Reason is not an option for such a setup. It's too much based upon using it standalone. Again, nice for drums, but that's really it.
Ow yeah, on a note, hardware multitrackers aren't any less of quality than your pc's dsp card. It's all A/D - D/A that determines the quality, it's cheaper to use the pc card but if in both cases it's says Apogee on the back of the expansion cards than they're both granted with suberb quality, if you catch me drift. About the functionality of both devices can be argued about ( especialy HD recorders have a different purpose since they're considered as portable devices, a pc is not ), but it wont make em obsolete yet. For the same reason I still like working with tape recorders like Fostex D16 or Sony reel2reel.