Tire advice

[x6369x]

Hello all,
I need some tire advice.
My wife and I purchased a 2004 X5 4.4i with sport package in July 2008 from Carmax.
When we got it, it came with brand new Pirelli Scorpion Zero Asimmetricos (255/50/19 & 285/45/19).
Well, a little over a year later and 19K miles, the front tires are needing replacing (right front is worn on the outside) and the rears have a bit of life left in them.
The specs for the Pirelli's show a treadwear rating of 300 for the fronts and 420 for the rears, so by doing simple math, the rears should last 40% longer than the fronts.
I realize there are factors that contribute to tire life, ie-tire pressure, alignment, driving style, etc.

I've had the alignment checked about 6 months ago and Big O set it to factory spec.
I had it re-checked last week and it's still within spec.
As you can see by the pics I attached the rears are wearing slightly on the inside, which is normal from what I've read.
The right front outside is bald, but the left front is not as bad.

In searching through all of the tire posts, I've come to a consensus that the Pirelli Asimmetrico and Michellin Latitudes seem to be the most recommended all season tires.
One thing I like about the Michellin's are that both the front and rears have a treadwear rating of 440, but the downside is that they cost about $300 more for a set on TireRack.com.

Here are my options ...
1. Replace just the front Pirelli's for now and replace the rears with Pirelli's later down the road. Is there anything bad to doing this? Will this affect the tranny at all?
I realize this locks me into Pirelli's, but I'm happy with them.
2. Replace all 4 with either Pirelli's or Michellin's. I'm leaning more towards ths Michellin's since they have the same treadwear rating.

Thanks in advance!