I have both, and both have similar features.
An AXBB-E with a UCSB breakout board on the 3rd port gives you about the same I/O as the UC300-UB1, for about $100 less.

If you need more I/O, you'd want the UB1 + UD1 daughterboard.

The UB1 has a few more features, like 3 relays, and differential step/direction. And it only requires a single 214V supply, where the AXBB needs both a 24V and a 5V power supply.
An 8 channel 24V relay board is only about $15 from China.