Performance in general
In general 18s for performance and 19s for looks.
That being said the amount of performance decline you will experience with 19s compared 18s will be small but measureable. The basic reason is weight and moment of intertia. Rotating mass in a vehicle is more of a performance hinderance than non rotating mass because it takes more energy to get a rotating mass translating and rotating than to simply get a non-rotating mass translating. This obsession with rotating mass can be seen in all sort of design areas and many in cars. Engine designers are quite keen to lower rotating bodies mass and moment as well. Next thing about an objects moment of inertia is that it rises very quickly with its size - as the square of it's radius in fact (meaning double the radius = 4 times larger moment). These priciples explain the extra work brakes have to do in decelerating a vehicle as well as the hinderance larger wheels play in its acceleration.
The next area the heavier wheels (and tires) hurt you is in suspension performance. Anything that moves with the wheels is part of the vehicles so called "unspring weight". High unsprung weight is a huge detriment to vehicle suspension performance because all of that mass has a high inertia and can not respond as quickly nor as effectively to roadway force and profile inputs as a low mass/low inertia system.
OK, enough technical blah, blah, blah. Some pratical information: Wheel weight varies significantly by manufacturer and design. It is quite possible for a nice, weight consciously designed, forged 19" to be significantly lighter than a crappy 18". However again, apples to apples, with a similar design the 18 will be lighter and a lower moment of inertia.
Also to back up this claim I specifically remember an article that tested 18s vs. 19s on the M3 and they did measure better acceleration, braking and ride with the 18s.
Last but not least I personally can not stand how much the 18s pale in comparison in the looks department to the 19s, so I will comprimise. I'll go for 19s but will consider weight in my criteria. However, if I can't find a design I really like to look at I will also just eat the performance differences.
Hope that all helps.