There are models that have knobs, I would look for those. Other things to look for is how many power levels there are, how low the power is before it cycles on/off (lower the better) and how quick those cycles are (faster the better).

Think of it as similar to a microwave. Having power levels 1-20 would give you much more control than one that only gave you 20%, 60% and 100% options. The power cycling is what a lot of microwaves do (except inverter models) - instead of reducing the power, it turns full power on and off. If you are trying to keep something at a consistent simmer that can be a problem, so you want a stove that will do that as little as possible, and if it needs to one that cycles as quickly as possible to reduce the temperature variations.