My thought was for mechanical. the bb will catch up and have more bacteria in the filter will compensate for a higher flow to level it all out
Absolutely, no doubt that is how I run my other 2 setups, it could end up being cheaper, this is my ideal solution. Any specific ones that would fit well?
I like to geek-out on stuff like this (I like running numbers) so I apologize in advance for the long response.
It's definitely an important decision; since our pumps are running 24/7, it's crucial to chose something that is efficient while still providing adequate flow.
Assuming you are running a gravity-fed filter where the pump doesn't need to battle a lot of head, it sounds like two Laguna Max Flo 4280s would be a pretty good fit. Both of them combined should push 6500GPH @ 5' head and draw only 320 watts. That's ~20GPH per watt, which is awesome.
Not sure about Canada, but here in the U.S. they sell for ~$250 each, so that would put them right at your budget. I've been using this pump on my setup for ~4 years and am really happy with it.
I saw the recommendation to run a high pressure 1.5hp pump. Assuming you are running a basic gravity fed filtration system (like most of us do), I would stay away from high pressure pumps and/or the Baldor 1.5hp. Those pumps are really inefficient; for the amount of watts they consume, they don't push very much water.
The 1.5HP Baldor draws 1,500 watts and provide a maximum flow of 9,200GPH. (Even if you assume it's putting out 9,200 GPH which it wouldn't be, that's only 6GPH per watt...remember the Laguna was at 20GPH/watt).
The only scenario where the 1.5HP pump would make sense is if you were running a sump in the basement and you needed a high pressure pump to push water up one floor or more. Otherwise, high pressure pumps are totally unnecessary and a huge waste of money...
Assuming you pay close to the national (U.S.) average of $.11/KWhr, the 1.5HP pump would add almost $120 every month to your electric bill ($1,440 per year).
Both Laguna's would add ~$25 every month (~$300 per year).
So, over the course of 10 years, going with the Laguna's would save you over $11,000!
The Laguna's are a win-win-win. They are less expensive, they will be much cheaper to run over time and you will have the redundancy of two pumps.
If you definitely want an external pump, going with two Reeflo Barracuda/Hammerhead Gold pumps would be the next best thing (efficiency of about 15 GPH per watt). Still not as efficient as the Lagunas, but much better than going with one huge pump.
Hope this helps.