Yes, you can go very wrong with either one.
Glock's grip angle can be a problem. Yes, yes, I know it can be overcome with training. But, training effectiveness just does not compare to starting with a pistol that you don't have that problem to start with. And, even after getting used to the Glock's grip angle, there is no guarantee that it's a permanent fix.
If you're sensitive to heavier trigger resistance and rough break, you may have to spend more time and expense on training with a Glock than M&P. If this sounds wierd, keep in mind that Glock's 5.5 lb trigger really is not 5.5 lb.
If you are the type who heavily relies on "tactile" reset, then you can go wrong with M&P. You might short stroke an M&P if you came from Glock. Training issue? Yes. But, a non-issue with a Glock. Why choose to have a training issue to overcome when you can avoid it? Unless there's some other gain that is.
If you need to have a light on a pistol rail, you can go very wrong with a either pistol. Glock still have not fixed the failure to feed problem with lights up to Gen 3, and Gen 4 still is an unknown. M&P has shorter rail, and some lights that fits on a Glock may not fit.
When someone says "You can't go wrong with either one." they're irrisponsibly ommitting the fine print:"...if you spend significant amount of time, expense, and headaches to overcome whatever charateristics you don't like about it."
Are they both accurate and reliable pistols? Yes. But, so are plenty of others. Merely pistols working as advertised does not mean it particularly suits you well.
Which is better depends on the user needs. One clear fact of the matter is that they are NOT the same.