Hi again.
I can't really say which is going to be best for your situation. Have a think about what you need in an interface, mainly the number of physical inputs you might need in the foreseeable future. It is always a good idea to get something a bit more expensive with better specifications than what you need right now so that you don't suddenly find yourself limited by your equipment again. EG; if you only need 1 input right now, get an interface with a minimum of 2 inputs. You then can start experimenting with recording stereo tracks later on without the need to buy another interface. Or get one with 4 inputs so you can really get stuck into multi-track recording.
Do some research into USB and Firewire (if you have firewire connections in your computer) interfaces and see which ones fall within your budget. Then start narrowing down your choices by looking at what features each offers. Read user
reviews on what they think of it, what issues have arisen and if the fix for the problem is within your means. You should then end up with maybe 2 or 3 suitable devices to really look hard at.
Then wait for a sale then see which of the three is the cheapest or has the most add-ons

(That's what I do, anyway)
When you have a converter that you know is not causing you any problems with capturing your audio signal, if your mixes still aren't up to scratch then you need to
start looking at your mixing technique and maybe your recording technique. Practice, practice, practice! Experience with mixing and finding solutions to your problems will make you a more competent engineer.
You'll also start discovering the limitations of whatever equipment you start out with (usually the cheapest available for many of us) and you'll then know *why* you might need something different.
Before too long you'll have the same gear-lust the rest of us have where we simply *must* buy audio gear because we know we have a need for it...or could find a use for it even if we don't need it right now
Good luck mate!
Dags