I have used both, and mostly stick to Flickr.
Flickr seems better to me as a 'sharing' site - you can easily upload a set of images, and can easily share them with friends and through the groups which gives you access to images and discussion on hundreds of topics - groups dedicated to certain cameras, lenses, styles, locations and many more where you can see what other people are doing in certain areas and the chance to discuss the topics with others of interest.
In those ways Deviantart seems a lot more limited - the upload system is a lot more cumbersome as you have to upload images individually and set the options each time, whereas Flickr gives you the batch controls. The upside for the Deviantart system is that you have more control, and the option for sales and so on, but the sharing aspect isn't really as prevalent.
Flickr does also ask that you pay for the pro account for full access to the options (without it you are limited to a set amount of albums and it only shows your last 200 uploads) - but it is fairly cheap and even with the cost I felt that Flickr was more appropriate for what I do.
