Well, Eheim makes a fantastic product so reliability and quality are not an issue here. My question for you is, what do you want to accomplish with this pump? There may be better suited pumps to the task
Well, even the compact 1000 only has 295gph at 6.6ft of head. Thats too under-performing for a sump return. Water will just trickle out the top by the time its lifted up. Those compact pumps are more used for reactors, not sump pumps . Go with the 1250
Yeah, much more stable with a pump. However, you mention that you're adjusting the drain? . What kind of drain are you using? You should have an on-demand gravity drain of some type for the safest operation. The drain should never be controlled, only the sump return.