Here is a page from SoundCloud help:
From that page:
What’s the best file I can upload?
We recommend you to upload in a lossless format like WAV, FLAC, AIFF, or ALAC. If you can, the bit depth and sample rate should be 16 bit and 48 kHz respectively.
When you upload a track on SoundCloud, we optimize it for streaming. As part of that process, we resample and transcode it to a high-performance codec. These codecs are highly optimized, but they can increase peak levels, which can cause clipping. We recommend that you leave between -0.5 and -1 dBFS of headroom to prevent artifacts like clipping during transcoding. Make sure to check the resulting stream in the rare case where more headroom is required.
I’m glad you asked this, because I have been uploading MP3s thinking that the size of my uploads would count against my time limit. But rereading this, it sounds like you have every reason to upload the WAV file of your mixes.
A couple more things - You can also enable “downloads” on your SoundCloud song so others can download it. Also, you can also keep your track “Private” so nobody can see it unless you share the link. I use this all the time so I can check mixes in the car on the way to work.