For me it really came down to size...the Tunze 6025 is a little smaller than a baseball, a lot smaller than I thought it would be. The K4's do have a rather large footprint (that egg gets big on the 4 series). Plus there are threads here on modding the Tunze 6025 that almost double the out of box flow rate (from 660GPH to ~1100 or so GPH) which I like...basically you can get a similar flow rate in a smaller package with the Tunze. When it comes down to it the K4's, Seio's or Tunze are all good "powerheads" and it should come down to your budget and personal taste as all of them will provide flow in your tank.
As far as the Maxijet, I've heard people refer the modded flow rate as LPH (liters per hour) which would be dramatically less in GPH...there are Maxijet mod nuts here that can tell you more than you ever wanted to know (check in the SPS, Advance Topics and/or Equiptment forums)...I am *NOT* one of those experts.
