Hi Holger,
I'm guessing that when you say "DigiRack" you probably mean one of the rack-mounted versions of the Digi002 or 003? Confusingly "DigiRack" is the name of the suite of software plug-ins that comes with all PT systems. In that case you would be using Pro Tools LE. There are two basic forms of PT, there's the LE version, which is cheaper and generally runs using just a fairly straightforward USB or FireWire interface and then there are TDM systems, the current version of which is called Pro Tools|HD. These are generally used either by professionals or by people (like myself) who've saved up a lot of money to buy one because we have delusions of grandeur.
There are some differences between the two software versions but nowhere near as much as they used to be and I don't think there's anything really important that's missing from the LE version.
As always with such things, it's very much a matter of personal taste. I've tried most of the major sequencer packages and although I can see why other people love Logic, Cubase, Digital Performer, Sonar, etc. etc. personally I prefer PT. This is probaby at least partly because I learned about recording in tape-based studios so I like to work primarily with audio and I like what MIDI (these days that means mainly the VIs!) I do use to be as closely integrated as possible. The great thing about PT is its simplicity. It has just 2 windows to navigate and you can do everything in those 2 windows. Basically most of the other sequencers I've mentioned started life as MIDI sequencers and then had audio grafted onto them once computers became powerful enough to handle it (mid-to-late nineties). With PT it was the other way around, it was an audio recording program to which MIDI was added not all that long ago.
Because I've had an HD system since 2000 I've never tried any of the LE ones but I believe things like the Mbox and 002/003 are excellent interfaces - whether you'll see an improvement in quality will depend on what you're using now. I saw a big difference because I went from using an interface costing maybe £300 (back then) to using a £6000 Pro Tools TDM system which came with pretty high-class converters, al least in comparison to my previous ones although you can go higher... But then one of the rules of equipment is that you can always go higher if you spend more money... Which is why most musicians are broke, of course. [:)]
So basically, unless you're thinking of upgrading by spending some serious money on a TDM system (and there are a few competing systems, I know people who swear by Soundscape, for instance, or there's Nuendo... And several others), which I don't think you are from what you say, it really depends on what equipment (hardware as well as software) you're using now and whether you're happy with your software. It sounds as though you're not. The PT software isn't to everyone's taste, of course, but if you liked the demo then chances are you'll like the full version.
Another advantage of an LE system is that it's really pretty compatible with full TDM systems. This means you can record a load of stuff and then to a professional studio and if they've a full TDM system (which many have) they'll be able to open your sessions. Can be very useful, that.
The RTAS VI for the Mac is now working very well indeed on my system and it's been an absolute godsend, I'm getting so much more done than before when I was having to run 2 separate Macs and transfer audio and MIDI between them. Thank God that's over and thank you VSL.
Anyway, I mustn't go on, I've got work I'm supposed to be doing to pay for all this kit... I hope this helps a bit.
Nick