The UB1 is the best board for the UC300ETH. It's basically an MB2 designed specifically for the UC300ETH. (The MB2 was designed for the Ethernet Smoothstepper)
With the UB1, you eliminate the need for the 5LPT motherboard.
Why the UB1? It's well built, and has a lot of features included. It uses 3 of the 5 ports on the UC300ETH, and has an optional daughterboard (UD-1) for the other 2 ports.
If you use other breakout boards, you'd need multiple boards to get what the UB1 gives you.
Your stepper drives will connect directly to the breakout board. They'll also require a DC power supply to power the stepper motors.