I don't think better is the correct word, both the Scar 17 and Spear are modernized, large frame AR18's,
I think that despite sigs history of shoddy qc and product abandonment, I think one is more likly to see interesting variants of the spear from sig, for example, I could see Sig do a rattler version of the spear in 8.6cm. That could be cool, I could see Sig release an accuracy barrel for the spear, where as FN doesn't care, or can't get acceptable accuracy out of the 17 and only offers that option in the form of a stand alone weapon system. Looking at the construction, it would appear that electro optics would fare better on the spear than the Scar.
I had a scar 17 and its not a bad weapon, its just the expedited result of a mid 2000's RFP that never really underwent a product improvement phase. Essentially the same mk17 that was field in 2008 is what you see now. Breaks, optics, horrible to suppress, plastic 1913 rail sections, weak stock hinge. If the speak, addresses these short comings, its could be interesting.