What are your export settings? You can export as Apple Lossless, WAV or AIFF if you want the best quality, but either way Soundcloud transcodes it into a lossy, more compressed format to make it more suitable for streaming (although the high quality file you uploaded initially will be the version users can download if that's enabled).
Also check to make sure you have Soundcloud turned up all of the way. On the top right corner of the page above the search field there is a volume icon that shows a slider when you click on it.
Basically you shouldn't be experiencing terrible issues with sound quality if you've tweaked the export settings appropriately.
Just to clarify before I make any suggestions is that what most DAWs call exporting isn't called exporting in Logic, it's called Bounce. If you use the export command in Logic it will give you seperate WAV files for each track and .midi files for any midi tracks you have in your project. The Bounce command is right below the export command in the file menu, though.
The settings you choose when bouncing really depends on your preferences and how big of a file you care to have. If you have no mastering or mixing left to do you should choose PCM saved as WAV or AIFF (I'm honestly not sure of the pros and cons of the two), but you can bounce the project as more than one file format at once so you don't really have to choose only one.
If you bounce to an MP3 go ahead and choose the highest settings which I believe is 320CBR (320 constant bitrate). You can also bounce as AAC, which I prefer because Apple Lossless results in a fairly small file (compared to WAV) while maintaining good quality. You can also manage the file's metadata from this menu so your ID3 tags are good to go.
Like I said it's really up to your preferences and what you are going to use the file for.