Repair my Lens or buy a new one?

If I can interject myself again (and I will) I do own the 24-105.

I have had the chance to test it on my 6D as well as my 60D, to which I compared the Sigma 17-50 both in regular photos and charts.

The Canon really can't hold a candle to the Sigma. In sharpness or in speed.

Assuming the 17-70 is close to the 17-50 (and I have every reason to believe they are very close indeed) as well as being made for APS-C (another advantage) then the Sigma is the obvious choice optically.

However, it's not that easy as you know.

The resale value of the Canon will be higher. The handling of the canon and overall feel are better (Silky would be an adjective I'd use), but the overall build quality is actually quite close. Weather sealing on the Canon also. And of course, if you ever go FF, you will have a serviceable lens moving forward.

The Sigma is a more modern design, and I'd also wager the OS is better than the Canon possibly by one stop. Great warranty too. It's faster on the wide end - and you will notice f2.8.

This is mostly conjecture on my part as I've not handled the lens nor even seen reviews on this lens it's so new - but based on the other recent Sigma additions. It could be that all of a sudden out of all the fantastic new lenses Sigma has developed that this one is a turkey. But, I kind of doubt it.

So, that might not have answered your question - but possibly given you more to consider?

I don't know what the prices are going for in the UK right now - here there are current rebates for the Sigmas. Canon is giving away a printer if you get a lens. The 24-105 sells individually for $1100 US, but I would knock off $300 at least because they're being sold as kit lenses - something you might look into over there as an option? Folks buy the kits and sell the white-box to lower the price of their bodies.

Good luck with your decision.