I collect both 1/18 and 1/43 (the popular scales).
Naturally I have more 1/43s although I started out with 1/18 and, at first, wasn't interested at all in the small ones because they weren't as impressive as their big brothers (I missed many great models in 1/43 because of this stupid policy of mine

).
But when I bought my first 1/43 (a Coulthard McLaren MP4/12 '97) I was hooked. These little models had remarkable quality for their size and also sometimes looked better than their 1/18 counterparts because they were, let's say, more 'solid' as structures.
Also, it is great that these small ones come with a nice plastic base (that can be used as a 'building block to create an entire 'wall' of models as well!) compared to the cheap looking box of the 1/18s (due to the cost of a big base I suppose).
Furthermore, the 1/43 scale has by far the greates selection of models and the most variations.
Plus, it is the price. For example, my local die-cast store has just received complete runs of the Ayrton Senna Collection in both 1/18 and 1/43. Although I would love to get my hands on those 1/18 Senna models, I can buy two and a half 1/43s for the price of one 1/18. So, guess what? First I buy as many 1/43s I can, and then if I have the money AND the 1/18s aren't sold out yet, I will go for them.