[Pumilo]

The sump pump that you linked to is oil filled and direct drive. Eventually, the seals have to fail. All that oil will then be in your tank. Pond and aquarium pumps are generally magnetic drive and either oil-less, or completely sealed and driven with magnets so that seal failure and oil contamination will not occur. Additionally, the sump pump is probably not built to be run continually. It could also be made of cheaper plastics that may not be completely safe to be used in a viv or fish tank.
More expensive, but quiet, high performance, able to handle the head height, and longer lasting, would be the PanWorld external pumps. I used one on my wife's reef tank to pump from the basement to the upstairs living room. I've used a lot of different pumps running a coral farm and let me tell you, this thing was QUIET! Looks like maybe the 50PX would suit your application. Here is a link for a good price. PanWorld 50PX External Aquarium Water Pump - 590gph - English Here is a link so you can look up the stats. Pan World Americas, Ltd. | The World's Best Magnetic Drive Centrifugal Pumps | PX Series

 

[Pumilo]

A maxi jet 1200 should work well. Make sure it is an OLD STYLE one the new " improved" ones click when they have back pressure on them(ie hose). The old ones are in a green box and do NOT come with a modding kit to up the flow and make it look like a koralia power head. Should be able to get one at petco ect. Hope that helps. Pretty quite I run 3 on my tanks.

A maxi jet 1200 tops out at 6 feet. He wants to run it at 5.5 to 6 feet. It would either barely trickle or not run at all.

 

[Pumilo]

How big is this waterfall your trying to run? 5-10 GPM is 300-60 GPH which is a lot of flow if your pushing it through a pipe smaller than 1" (common garden hose is generally 5/8", and most barb fitting aquarium pumps are 1/2"). For comparison, a standard shower head produced in the last 10 years will most likely have a flow rate of 2.5 GPM.

What is the width of your waterfall, and what are you planning on using to spread the flow across the width of your fall?

The Pan World pump that Pumilo linked is a really decent pump and will give you 6-7 GPM at 6 ft of head height, but I still think that this is more flow than you're probably looking for. I actually like Iwaki pumps a little better than the Pan World, but unless you really need the head height or large volume flow, the Pan worlds are probably a better fit for this situation.

Both pumps you linked to are submersible internal pumps, what size enclosure is this waterfall going to be in? Do you need to use a submersible or would drilling a bulkhead for an external pump be an option in your build?

I design and install aquariums and ponds for a living, sizing pumps is a regular activity for me.

As to your question of noise, I'm NOT saying you have to go buy the most expensive, top of the line pump, but generally the cheaper you go, the more noise you will experience, especially as the pump ages.

I agree a Japanese Iwaki would be better but I'm not so sure about the American models. I was trying to get a quality pump without going nuts on the price. I agree too that his specs sound like a lot of flow and he may not need as much as he thinks.

 

[Pumilo]

I've not used the American Iwakis yet, but I can't imagine that they're as good as the Japanese Iwakis.

They each have completely different motors, while the pumping section is the same.