I have to respectfully disagree with a lot of what is being recommended here.
First, I love RME. However, the RME converters in the Multiface are just barely a step up from those in the MOTU units. A pair of Fireface 800's may do the trick though.
Second, not everything Bob Katz says is gold. Not only this, but there have actually been many advancements in technology since his book was written. I am not going to sit here and debate Bob, but his opinion on this issue is vastly a minority opinion. When you get to stuff like Prism dream converters, Weiss and Lavry gold, then maybe the Big Ben will not make an improvement, but then you are also spending more than $1000 per channel of conversion.
The aurora 16 channel with a Big Ben is closer to $150 or less per channel but still offers a noticable improvement over other similar priced and less expensive converter setups.
Third, bumping over to digital will not give the same analog sound. Each different stage of analog offers different advantages and disadvantages. A certain degree of that sound will be maintained by tracking to tape, but after the conversion to digital the process will seperate. There is still something different about analog EQ upon mixdown in combination with analog panning and summing. I am not trying to say one is better, but the difference is there none the less. I would not however be wanting to rely on the cheap converters in a Roland VS machine and definately would not consider the mastering suites to be great. Unusable is how i would see them. At least with a DAW there are more options available to configure a system that meets anyones needs.
As far as the differences in the sounds of the converters... The differences can be minor and can be fairly major and those differences depend on many factors. Here is what I have noticed with using nicer converters. First, tracks seem to have smoother yet more range in the ends of the frequency spectrum. Tighter punchier lows, more extended yet silkier highs. Of course these are all "buzz words" but I do not know how else to describe it. I have also noticed that better converters seem to relate a much wider and more 3 dimensional depth to a track. Even mono tracks seem to have a higher degree of realism conveyed. Once you start mixing, better converters seem to offer both a wider and a deeper soundscape, at least in the listening environment on the D/A side. These differences may seem minor on some source signals, and more prominent on others. The difference often becomes more noticable as you get more and more tracks going simultaneously, and often can be felt in how your mixdowns form easier and smoother. Digital clocks are a whole different ball game. Without going into the details, adding a better clock makes your converters a little more stable which in turn helps them to function closer to their full potential.
Without knowing more of a budget, it is really hard for me to tell you what I think may be the best choice for you. Since you mentioned both SSL and Motu, I felt like Aurora was a decent compromise between price, feature set, and quality.
AIn any event adding a Big Ben clock to the converters (about $1300) will make a noticable improvement.
