Which should I collect 1:18 or 1:24?

It is a popular debate among the diecast model collectors.I am just pointing out the pros and cons of each scale,hope it will help you!
1:24 Scale:
1:24 scale is a traditional scale (proportion) for models and miniatures, in which 24 units (such as inches or centimeters) on the original is represented by one unit on the model. Another way one may understand scale, is if viewing a 1:24 car from a foot away, it will be as if seeing the full size vehicle from 24 feet (7.3 m) away. It is also known as "half-inch scale", since 1/2-inch equals a foot. Countries using the metric system generally prefer to use 1:25 scale, which produces a very slightly smaller model, and in informal usage the term "1:24 scale" is often used to include both.
1:24 is a very popular size for Die-cast toy vehicles, which are collected by children and adults. Primarily automobiles are made in this scale, with a few examples of tractor trailers and other larger equipment. Plastic automobile kits are frequently made in this scale.
1:24 is the largest of the traditional slot car sizes - and the earliest. Lionel's (USA) 1:24 electric autos of 1912-1916 became the first known commercial slot cars.[1] In 1955, the Model Automobile Racing Association of Kalamazoo, Michigan, built the first track for electric rail-racing (the short-lived immediate predecessor to slot racing) in the US. Unlike the seminal Southport (UK) track that inspired it, the MARA table was designed for 1:24 as well as 1:32 competition.[2] With that beginning, the US adopted 1:24 as the primary scale for serious competition during the heyday of slot car racing in the 1960s,[3] while Britain and Europe favored 1:32, and have continued to do so.(Source Wikipedia)
+Easy to find
+Low cost
+wide range of models
+Take less space for display
-Less detailed than usuall 1:18 models
1:18 Scale Model:
1:18 scale is for the serious collectors.There is no detailed history available for 1:18 scale models as,this scale has found market very recently.
+Very detailed parts,working steering(may work in some 1:24 model),openable boot lid.
-Difficult to find all the desired models
-Costly

Lastly,donot mix up scales,it will be less apealing for your showcase to have a buch of 1:64,1:24,1:18 models.If you have already mixed up,try to arrange different showcases for them.