I think I'd rather keep Horford, but if the salary difference would allow us to keep Smart rather than let him walk next summer (save a few million/yr), then I would certainly be up for a 1 for 1 swap.
If we are going to sign Smart anyway or if Mem would require additional assets, I would pass.
As Dino mentioned, there is no conceivable scenario where we can keep Al and not trade away multiple very good players on value contracts.