major issue I have seen with 5.0 and 5.1a are timeout errors when logging in to the vSphere Client and this has been addressed with 5.1b.
you are good to go with upgrade to 5.1b as its straight forward upgrade from 5.x versions. but I suggest you to have plenty of memory on vcenter server as all 5.x versions of vcenter servers are very thirsty for memory. update manager is not changed in 5.1b so you can still continue with same update manager bundled with 5.1. Dont panic at the screen while upgrading vcenter server product as "Installing Orchestrator" takes hell lot of time..approx 40 mins.