I am no expert when it comes to hardware but I can tell you straight off never use anything that says M-Audio (lol)....
Jokes aside...
I need to upgrade my interface myself. I was told scarlett focusrite is one of the best ways to go if you are on a tight budget. I am assuming thats 1500 USD you mentioned there which is more than enough to get you a decent setup. I think these focusrites start from about 200 USD. I always prefer something with at least 2 xlr/trs ports so that I have the option of running two mics at once or even recording instruments like an electric guitar (some interfaces allow both line and instrument level inputs).
But honestly, if what you have sounds good why spend money to upgrade? So what you should ask yourself is am I satisfied with the sound I am getting. Part of being a good engineer is knowing how to make the best out of what is available to you. When I started recording for other people I used an AKG dynamic mic. I did not like the sound I was getting out of it in some situations and no matter how I tried to manipulate it, I was not happy. I eventually was able to get a condenser (Rode NT1A) which is more suited for vocals. Plus I now have the option of switching between mics and experimenting with different sounds. If you want a decent mic I'd honestly go with a Rode condenser. They are one of the best and their designs are simple but effective. You can get one for about 300 USD. The sound you get out of a Rode is as close as you'd get to hearing the artist speaking directly into your ear.
Also see if you can test other interfaces with the mic you already have. If you are getting the same sound then change your mic. If you are getting a better or worse sound, then change your interface.
Also consider your soundcard. If you are using a usb interface with poorly designed drivers and converters then you should definitely consider your options there. Unfortunately, I am not knowledgeable enough in that area to give any suggestions. I need to do some research to upgrade my rig as well.
