Based on the fact that I just serviced the LX hubs on my wife's commuter bike, and the XT hubs on my touring bike this month; I think the XT hubs have more efficient seals. This is also based on the experience I've had servicing both the LX and the XT hubs several times. Purely anecdotal, but I do learn from my experience.

13-3/16" bearings or 9-1/4"- Is this important?

I just completed a fully loaded 2900 mile tour which included over 400 miles of unpaved dirt and gravel roads and trails, And over 500 miles of cobble stone and paver block roads and trails. I really never gave a thought to whether my XT rear hub had 9 big bearings or 13 smaller ones. The hubs did just fine. When I cleaned the hubs earlier this month, the grease was still clean and doing its job. That was in spite of riding 35 days in the rain, some of it on those dirt roads. However, I was a little anxious about the aluminum axle. I won't be concerned about it on my next tour.
I also use XT cassettes on our touring bikes, our commuter bikes and on my cyclocross bike. No issues with cog wear or free hubs. I know it is not statistically valid, but I'll continue to use them.