This is some good usb audio interfaces, which i found catchy.
Please help me in taking decision which one should i go for?

1.I like interface of Focusrite Scarlett, however after reading many reviews people are claiming about, as there is no MIX option (MIX" knob, which allows to control the volume of the Instrumental (Sound from PC) and the volume of the Microphone (Volume Into PC) to your headphones.)

Actually, i m a CLUB HOUSE, ELECTRO HOUSE, Producer and currently using FL STUDIO BUNDLE WITH ABLETON LIVE, .
So wonder which one is appropriate for me and please write down, pros and cons of each, i need your suggestion, as i cannot currently test them as they are not available in my country and going to purchase online.
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In addition, what is DSP and is it important to have or it has not importance in producing quality sound?

Kind Regards
Levan

anyone?

In context of soundcard, DSP means it has a processor helping you're PC CPU with some tasks, so you can run let's say EQ or compressor on the card itself. I'd say it's more usable when you're recording.

I feel like upgrading soundcard in this range too, so I'm actually interested in answer to you're first answer as well.

scarlett is better for recording due to better preamps, but has less features compared to the m-audios you listed, so all depends on what yo want to do with them.

Thank you for your messages, mates

Well as i said above, i prod electro, club house, tech house tracks, so i am not going to record guitar or vocals, i want for KRK Studio monitors, and using FL STUDIO bundle with Ableton Live.

I do not want to waste my money, if there is no such a difference among them, so if Focusrite can handle all jobs, what i want, i should go for Scarlett, but i have read it has lack of I/O. i'm still confused

I guess most?/some? M-Audio Fast Track Pros (incl. mine) has a bad habit to occasionally disconnect after minor current disturbances, caused by switching lights on, etc. (restart of soundcard needed!) There is a workaround solution I might try someday (includes mods that voids warranty!!), so therefor my vote goes to any other than M-Audio Fast Track Pro candidate!

Thanks for replying mate,

This is only con, that can be taken into consideration?

I've been using a Fast Track Pro for around 6 years. 128 samples, 5 ms latency, no functional problems with the x86 32-bit driver.

I'm not really a yesman, you might want to try each company's official forum.