In my opinion, there are no negatives to running a sump, just a very long "PROs" list:
- cheaper to build than a canister filter (use old tank or storage bin)
- more room for bio media
- BEST mechanical filtration you can get by incorporating filter socks
- remove all equipment from the tank
- maintains constant water level in the tank, and has extra water for evaporation
- keeps the surface free of scum via the skimmer/overflow
- can be made to run even quieter than a canister filter (Durso standpipe, Herbie method, etc)
I love sumps, so much that I have a 10g sump on my 57g planted with a 1,017gph pump, and the flow is just about right. For my tanks, I try to do 8-10x turnover minimum. You WILL NOT have too much flow at 10x turnover. Anybody who has ran big tanks will tell you this. You will barely even notice a current in the tank at 10x.
Here's the sump on my little planted tank:
