Well, that might be a hard choice to make! The 2305 Deere is great for blowing snow, especially with 4X4 and the rear mount 48" blower, the verdict is out on how it does cutting grass as I just got the 2305 only 3 weeks ago, and will be a few more months till I cut grass with it. I am sure I will enjoy cutting grass with it as well.
I love my 2007 Deere X744 AWS with 48" deck, really manuvers into tight spaces and has cut out a lot of unessary triming. I really like the features on the X744, has the same foot petal set up as your X520 plus very bright headlights and does a great job cutting. The X744 is really easy to service as well with the one piece hood simular to your X520.
I love my 455AWS as well, but is getting up there in age yet it only has 1205 hrs on it. So I guess I love all my mowers. The F1145 Deere likely will go for sale in the spring... Maybe... we will see.
Take Care Jason B