I ran a 4600 in my B4 on a small sticky indoor track once. I geared it 25/81 and it would still pull the front off the dirt coming out of corners, and obviously had tons of top end with that gearing. Doubt that a 10.5 could come close to it.
But a 10.5 is a good buggy motor for a small track, especially for a beginner. Liek others said it's a more mild motor... doesn't have the rip that a 4600 has.
But if you are asking which is faster? 4600 no question
Castle's motors are a different animal than other brands... can't really compare kv to kv.