From a speed point of view, your Computer is most likely much faster than the server you have for your website/web-application, so it would make more sense to resize on the client, than on the server.
I have a Java application where I change certain color pixels to another color, and then size the image down, and that total operation takes about 0.005 seconds per image to convert.
Depending on what your app does, it might be too much work to do on the server( in the case of many users taxing the server resources).
I would say that if you want to keep the load off of the server, and your server doesn't have much power, I would do it on the client.
If you have a server that can handle a large load (if you need it), and has enough power to do many conversions, then do it on the server.
Overall, the answer comes down to trying it out for yourself, test your environment, and see what works best for you.
You might find that you can upload a million images, from 10 thousand different users concurrently, and your server laughs at it....
Or you find that you to upload 100 images, by yourself, and your server starts to cry.
Do you need it for many users? Just yourself? You didn't provide enough information to say, so once you do, I will update my answer...
Hope this helps.
Most likely when you send the image to the server for resizing, you will have to
first save the image, read the image on the server,
delete the old
save the new.
if it's all done on the client, then you only will be saving the image once.