I will tell you right now to use the union valve 100%... If you look at the design without the valve, ask yourself how you will get the pump off for maintenance...
Also you may want to consider a short section of the 2" spaflex hose to help with vibrations from the pump. It will greatly reduce the noise it makes as the pump is isolated a bit from the rest of the sump.
Also I have a friend here who is a reefer as well and he hates the fact that his return pump feed plumbing has no shutoff valve. Every time he takes the pump off he has to first empty the return section of the sump or find something to plug the bulkhead with.