I have both and offer my opinion:
1:18th - large enough to handle and look really cool.  Do not look quite as toy-like.
1:24 - look fragile to me.  Different from the 1:18 in that the doors open, trunk opens, floor mats in the trunk...about the same detail just more moving parts. 
You won't be unhappy with either.  I prefer the 1:18th