I like the Dell 15z quite a bit and the Lenovo Z400 is a solid machine. I really haven't used the Acer M5 enough to pass judgement on it. And I hate to say it, but there are probably 100 laptops with 10 strong ones that meet your criteria if high portability isn't a factor. Asus has some lovely mid-priced laptops for example, as does Sony (though Sony's tend to be a bit higher priced). The Lenovo U430 might also be worth a look-- we have that in house for review soon.
-------------------- Lisa Gade Editor in Chief, MobileTechReview