The bigger the water softener, the more it will flow for a given pressure drop. That, to me, is more important than the grains rating. You can regenerate a small softener more often to handle harder water, but there's no way to improve its flow rate without upgrading the tank.
You definitely want a demand softener; otherwise you're wasting salt. The supplier should set up the valve to only regenerate about 50 percent of the tanks capacity; that ensures that no salt goes to waste.
Take a water sample to any water conditioning supplier and have it tested for hardness and iron. That will tell you what capacity you need and help you adjust the softener you buy.
Several years ago I bought a system from Ohio Pure Water. It uses a Fleck demand control valve and has been pretty much trouble-free.