I say if you like the SD that much better, by all means go for it, I'd love to have one myself.
As far as the trigger goes, what the LGS told you is total BS (imagine that from a gun store).
The SD trigger is, according to the manufacturer approximently 8lbs vs. the 10-12 lbs of the sigma.
It is however much, much more like the sigma than it is the M&P.
I think it would be a teriffic gun to have knowing of the improvements it has vs. the sigma.
Let us know if you get it.