I think the Magnums are excellent, reliable, strong as any FX engines. I now own 5 from .25 to .91, the .91 being my favorite. I don't own a YS, so I can't compare. My .91 will swing a MA 14x6 "K" at 10.5K with 5%Nitro, 18%(50/50) with the stock exhaust, and 12K with a mousse can pipe. The plane I have it in is a SPAD Fun-Fly weighing 8.75 lbs. It easily hovers, and will rocket out of sight until you can't see it anymore.
The Mag .91 is in a .61 case, and the 1.08 is in a 120 case. So the .91 is the way to go IMO. On the Fun-Fly, I have a 16oz tank, and have the timer set for 25 min, and land with plenty left over.
IMO the Magnums are a best buy!
You can save a few $ and get an ASP series 3:
http://www.goldscallop.com/viewitem.asp?idproduct=220
They're supposed to be identical.

I'd like to see some YS rpm figures if anyone has them, I'm told they have loads of power.