Hoi there

I've had experience with both, but don't own either.
Sigma 17-70:
- Decent midrange zoom
- Wierd macro - it focuses approx 2" in front of the glass - you won't be sneaking up on anything!
- Fast focus
- Good range - 17 is wide, 70 is fairly telephoto
- OK speed - f2.8 doesn't last long. f3.5 isn't all that fast...
- Decent sharpness, nothing amazingly special wide open.
Canon 50:
- Prime lens - no zoom - one focal length only
- Excellent image quality with creamy bokkeh
- Stunning autofocus speed - even in the dark
- 50mm (75mm equivalent) is not wide. If this is your only lens this could be a problem.
Primes are great. They're fast, give amazing image quality and force you to really think about your shots. They can be small and lightweight (exotic ones aren't). But to gain a complete set of focal lengths you'll need a whole bag full - I currently carry 4 around with me - 20, 35, 50, 135.
I shot a festival recently. One guy with a £2500 camera, one of the Canon pro models, had a 50 1.8 welded on throughout, despite a selection of Canon L zooms in his bag. On his second camera hung an 85mm prime. At the same event out of 8 shooters, 6 were shooting with primes - mostly 50s. They're simply the most cost effective way of getting the speed and quality required.
My reccomendation is to go with the 50 and see what you want to do from there. Others will reccomend the zoom and I have no problem with that - zooms nowdays can almost match the quality of primes and are much more convenient.
But that 50 is a lens you should have anyway and it's a fine way to start learning about photography - having 1 focal length only will force you to be really active with your composition. You'll use your body a lot more, zoom with your feet, lean in and out. And for those who say landscapes will be rules out until you buy something else, my best ever landscape shot was taken at 50mm.