I'm assuming you have two overflow kits, one for each overflow.
From what I've found they rate at 600gph each so you are looking at 1200gph FROM the tank. You need to make sure whatever pump you get is LESS than the flow from the tank. Look at gph at about a 4ft head when shopping for pumps. If your pump exceeds your overflow your tank will flood.
I prefer pvc over flex tube, if you use the seach tab here you'll find varying opinions and pros and cons. If you are not handy at all flex tube may be the way to go.
If you go with flex don't let it sag like it does in hrandogs pics.
Overflows work on siphon and gravity, why make water flow uphill after drawing it down?