those questions usually get over 50 replies. i think they should be there to narrow down the search a bit. let's say, from what's the best trainer to what's the best gas trainer with a 50 inch wingspan that will fit my .46. the faq could say:

your average trainers are .40 size. that means that they are designed for an engine with a displacement of about .40 cubic inches. they are commonly referred to as 40s. 40 size trainers have a wingspan of around 50-55. there are other sizes too; 20s, and 60s and even a couple 120s (1.2 cubic inches) but most are 40 size. they are a good balance between size and cost. they usualy cost between $60 and $100. most trainers are about the same. the cost depends on the brand and the quality of the cuts and materials used. some common characteristics of trainers are high wings, dihedral, (wings have a slightly up angle) low wing loading (a small amount of weight for every square foot of wing surface) and a flat bottom airfoil.