Solid vs vented
There is no difference in the stopping distance from solid and vented discs the first time around.
Where vented discs come into their own, is there enhanced ability to get rid of the heat generated during repeated braking. So after a few hard stops the solid discs will be getting very hot and their braking ability decreases. The vented discs will get rid of the heat faster and so will take more abuse before they overheat.
Bigger is better?
The bigger the disc the more stopping power it has in general. The further out from the centre of the wheel the disc is, the easier it is to stop the wheel. Also as a side effect they are bigger so can get rid of heat more easily. However there are drawbacks, big discs need bigger wheels to fit under. For rover 200/400 the 262mm discs will only fit under one type of standard 15 inch rover wheel without spacers, the rest will foul the calipers. They also add unsprung mass to the wheel so can hamper handling to some extent but in general this isn’t an issue you’ll need to worry about on a fairly standard car.
You’ll see all sorts of sporty looking discs with grooves slots and holes in them. Are they worth paying the extra for? Well it depends on what you are after. In general avoid drilled discs, as they have been known to crack around the drilled holes, (I’ve personal experience of this and I’ll try to include a picture when I get one). The purpose of the grooves and slots and holes, (apart from looking good), is twofold.
First it is to allow hot gases generated by the pads to escape when braking. If they do not escape then the pad tends to skate over the disc surface on a cushion of gas rather than grip the disc properly. The gases are usually generated when the pad material is getting too hot and so this is what gives the brake fade where you push the pedal hard and the brakes have little effect. So they will help reduce brake fade slightly, but it alleviates the symptoms rather than treating the cause so better pads may be more worthwhile.
Second is to prevent the pads glazing. The outermost surface of the disc brushes lightly across the disc surface all the time. Over time this surface tends to glaze over and this leaves a tiny layer of pad material that doesn’t brake effectively. The grooves and slots tend to remove this surface layer and so the pads don’t tend to glaze as much. As a side effect though the pads will wear faster. Another drawback of slots and grooves is the extra noise when braking hard. Whilst this probably won’t worry you it is worth mentioning that they tend to “whir” when braking hard from speed – this is normal.