The flow rate really depends on the pipe size more than anything.
- Can be added to existing tank without drilling.
- Easily movable.
Internal overflow through the bottom
- Easier to hide than a HOB
- Simpler to plumb than a through the side overflow.
Internal overflow through the side
- Less likely to get blocked than an internal through the bottom overflow.
- More difficult to hide the plumbing.
- Theoretically should flow less than a through the bottom overflow of the same diameter due to slightly less head pressure. May not be applicable in real life.