TK at Kaiser Early Childhood Education Center

Our son will turn 5 in November and he is eligible for our neighborhood school's TK program (Hillcrest). The TK program will be at a different location,  Kaiser Early Childhood Education Center , where it will be pooled with the 2 other TK programs (Chabot, Peralta). it seems like it will be a big transition for one year of TK, followed by another big transition the following year when he begins K-5 (another new location, new friends, etc). 

Would love to hear from other parents who have had experience with Kaiser TK about their experience. Any pros/cons of TK overall are welcome too.. Did you find your kids grew more than they would've at their preschools, was it worth the extra transition? 

We are considering keeping him at his preschool one more year mostly because we feel bad uprooting him from his friends (once for TK, then again for K-5), plus we are planning to enroll his younger sibling at his current preschool this fall which would make logistics easier if he stays. But we are also worried he might get bored with one more year at his Montessori when he's the oldest next year (ages 3-5). Thanks!