I like having a WeTransfer channel for incoming files from clients. It's super easy, you can make the page look nice/modern, and the upload/download notifcations are great:
Clients get taken to the upload page after filling out a new project form on my website and you can also provide a direct link.
I like using Dropbox for outgoing files because you can just paste the file(s) in your Dropbox sync folder, and files upload in the background. I'm not sure about Windows but on Mac, you can right click on the file while it is uploadin and get the download link right then and there, and then paste it into your outgoing email to your cleint. I even use Mail Butler for Mac to delay when the email sends to so the file(s) is fully uplaoded by then. I very rarely have to go on the Dropbox website and mess with sharing folders etc., I find that method to be more of a hassle. I just share zip files of various things which is more direct IMO.
Dropbox lets you keep files and download links active for a long time for "those" clients that come back to you weeks or months later looking for their files again because they weren't careful. You can just say, it's the same download link(s) as before. Check your email and download again. I even have an add-on service that lets me restore really old deleted files and usually the download links remain the same, or worst case you make new ones. I like the user management tools of Dropbox but it can get messy if you're one of thoes users that sets up folders to share. I just make a folder for each month and then only sync the current and previous months to my computer. The old months get unsynced from the computer to free up local storage space but are still active on the Dropbox website if needed, and clients can still download the files. Right now a 1TB Dropbox account is pretty afffordable. It used to be only 100GB for the same monthly/yearly price.
There is also a new service called Fidbak that does a few things such as let clients upload new files to you (similar to WeTransfer channel), and it also lets you post WAV or mp3 files for clients to approve, as well as host zip files such a DDP image, or just a zipped folder of WAVs. You could also post a PDF or PQ sheet, it doesn't have to be audio files per se.
I'm testing the waters with it but not ready to use it for everything. It's a nicer option than SoundCloud for those clients that want to be able to listen to something via web browser.