How much are the weight difference, that's my main concern.
Or should I just keep my 18 rims, then sell it later on and pay some extra for more exotic 19 inch rims?
The weight difference is virtually non-existant. I'd suggest you choose the ones you like best. IMO the 18" rims look best and if there are aftermarket rims that look way better than the stock 18" not to mention then 19" OEM rims.
Some people say that, the 19" wheels give better handling. I have only tried them on streets so I can't comment on that but if your going to track your car, you're way better off with at separate set of track wheels with R-tires or even slicks.