I like Morneau and I followed him a lot the last few years. The biggest thing to watch out for his is BA. He'll get his power numbers, he can really lay into something if he gets it; but he could be a liability in the BA department. I think he was hitting in the low-mid .200's when at AAA this last time. Homers every night, mind you, but not so much of an average.
Lowell is a better all-around hitter. Considering there will be some adjustment time for Morneau, I'd definitely stick with Lowell. Although in the end, Morneau may actually have a few more homers, probably not as many RBIs.