Performance wise the xl will give you more torque and tend to run a little cooler. The 9 will have more power than the 10 but less run time as a rule. Here is BK's web site to get more specs.
Get a temp gun (cost about $15). Ideal motor temps are 160F, anthing less is cool - There is a thread in the FAQ forum that is very useful on temps. The wanderers have 5.5mm gold connectors (Mike sells them here)