First of all, don't upgrade your gear unless you know why you are doing it. 'Because I like playing with new gadgets' is a perfectly good (well, maybe not so good), and very common reason, but instead I'll concentrate on possible reasons which have some photographic aspect.

There are, really two of these:

1. because the lens you have is not good enough quality;
2. because the lens you have won't do something you need it to do apart from in the limited sense of raw quality.

I will disregard (1) because I am quite sure that the 50/1.8 Canon lens is perfectly fine. People obsess both about absolute lens quality and about the magic qualities of certain lenses and like to think that these qualities will somehow make a huge difference to their photographs. Almost always they won't. And before you dismiss me as not understanding lenses: I have a rather substantial collection of lenses some of which are claimed to have near-mystical qualities (and in some cases actually mystical qualities), and at least one of which I had specially fabricated by gnomes from sand imported from the Moon. I love them, especially the lunar ones, but they just don't make me a great photographer.

So that leaves (2): will the new lens do something photographically interesting the old one won't? Well, yes, it will:

1. it is about half a stop faster;
2. it is wider – its field of view is different;
3. ... and there may be some other features associated with this specific lens which I don't know about.

For (1): half a stop isn't much: you won't get shallower DoF really as the lens is also wider, and it's just not that much of a gain. Don't buy it for this.

For (3) I don't know. Perhaps there are some considerations specific to this particular lens and/or camera combination.

But (2) is the big question, and where it comes to knowing why you want it.

The 50mm lens you have is a mildly long lens on a crop-sensor camera. It is probably a lovely lens for portraits, but it's rather long for the way people tend to use lenses for, say, street or environmental photography.

The 35mm lens is going to be much closer to the traditional 'standard' field of view. It is still quite a lot longer than a lot of people use (Canon's APS-C crop factor is 1.6X, not 1.5X like most everyone else), but a lot of cameras used to come with lenses of 55mm which (remember this was for 35mm film) has about the same field of view as this lens when used on a crop body.

So, well, what do you want to be able to do? If what you want to do is make portraits of people then the 50mm you have is probably close to ideal. If what you want to be able to do is more like street photography, then the 35mm may be better, but you might also consider something wider: With Canon's 1.6X crop factor a 24mm lens would be about equivalent to 35-40mm on full frame in terms of field of view, and this is a lovely length.

So, as I said, it depends on what you want to do, and only you can answer that question. In particular the 35mm lens is not just a better version of the lens you have: even if it is better, it's not the same field of view, and would want to be used in a different way.