I think Toffoli is a perfect addition for the Flames, as prior to the trade they had effective first, second and fourth lines, however the third line was a real weak link. This trade should give them a chance to roll four effective lines. An important factor here is that Toffoli is under contract for 2.5 years at a very reasonable cap hit, and he's shown no obvious signs of decline at this point. This is far from giving up a ton of assets on a rental.
I just don't view Heineman as being a great prospect, I've seen it suggested he may only have fallen into the 7-8 range in the Flames minor league system, which while it's been improving lately is still a bit of a weak system.