I haven't used the Onyx i series of mixers, but I did have the top of the line Onyx 1200F 12 channel firewire interface (now discontinued) and it was AWESOME! And I now own the 003R+. The converters in the Mackie were definitely pleasant sounding but really what I think is going to make the biggest difference in terms of the sound you get between the Mackie and Digi interfaces are the pre's. In most cases I'm using some nice outboard pre's so I don't often use the pre's on the Digi, but when I do, I'm sometimes reminded of why I liked my Mackie Onyx so much! The pre's were very open and transparent and they handled transients and harmonics like nobody's business! Granted the 1200F that I had was a 2,000 unit when I bought it and the pre's on the new, less expensive mixers probably aren't as nice, if the pre's on the new Onyx boards are anywhere close to the older Onyx models, they'll still sound pretty nice (but maybe not as nice as the 003+).
Now there are a couple more things to keep in mind: firstly I know this thread is about the converters and I've been talking about the pre's, but for what you're doing I'd put getting good pre's higher on the list of what's going to affect your sound quality that great converters. Don't get me wrong, having good converters is important, but at the stage you're at, I don't think it's as important as having some great pre's IMHO.
Second, keep in mind that if you get the Onyx and you want to use Pro Tools, you're going to be limited to using Pro Tools M-Powered and while it's an ok program, It's more limited than pro tools LE (which can be expanded with more tracks and options with the Music Production and DV Toolkits). Thirdly, while Pro Tools LE is included with your 003
, you'd have to go buy M-Powered on your own and that's another $200 you'd have to spend right there.
To me, if the 003+ is in your budget, I'd probably go for that. Even though my Mackie had great pre's, I'm gonna guess that they're not as nice as the ones that are currently on the new lower end Onyx line. Honestly, some of the lower end Mackie pre's don't really sound as good as the Digi pre's.
And I think you'll be happy with the conversion on either!
Just my $0.02