tele for E-M5
I've had both, and used both on my OM-D.
The 45-175mm is a tad longer, but it doesn't extend when zoomed; the 40-150mm extends a lot.
They weigh about the same.
The reversible lens hood on the 45-175mm is smaller and makes the stored lens more compact than the Oly.
The 45-175mm has OIS, which is very useful at these FLs with a Panny body (no IBIS). With the Oly this isn't necessary, but it does make the lens more useable should you acquire a Panasonic body in the future.
The 45-175mm has a power-zoom button you can use if you want (or just use the barrel if you don't). This makes it better for video (I'm told), but since I don't shoot video I can't comment on this.
Some people have reported double images (blurring) with the 45-175mm at shutter speeds between 1/60-1/200. I haven't had this problem, but definitely search on this topic.
After receiving my 45-175mm I tested them against each other and decided that there was little difference optically. The 45-175mm was a bit sharper, with better contrast and more vivid colors (though by no means was the 40-150mm poor). Now that I've been using the lens for a while I'm glad I kept it; images have a bit more pop.