Soundwise it's pretty much a wash. Both choices will drive your speakers quite well. It probably comes down to cost and fitment within your rack or such...
You might want to check out the newer D-Sonic class D switching amplifiers on your magnepans. As you probably know, magnepans sound best with plenty of power. I have the same brand. but much older ones, and can attest to this. But I agree with both Wainwright and Tan43 comments that mono blocks are generally preferable and that 50 extra watts is not going to make a significant sonic difference.
The D-sonics are a newer design that 6moons.com reveiwed very favorably recently. They're small, run cool, can handle down to 2 ohm speaker loads and supposedly have good ss bass with tube-like qualities in the midrange on up. They seem too good to be true but offer a 21-ay free in home trial with money back guarantee so it's hard to lose much. They offer the normal B&O Ice Module amps along with amps based on the new module design that are purchased from a European company but assembled and sold in Houston.
I have no affiliation with the company and I'm not a dealer or salesman. I'm just very curious and would love to hear these on my spkrs and thought you might want to give them a try, or atleast a read. Good luck.
One more alternative that I just bought: the ClassD Audio 440SC (or 470 amp). Supposedly similar to the D-Sonic sound for less dough. I haven't heard this yet but am taking a chance based on word of mouth on several forum sites. It's not a big chance since it comes with a trial period. You can check them out at classdaudio.com.