There's been some confusion on this issue. In a sedan, with 5 or 6 cells, the 13.5 is probably the closest thing to a "stock" motor out of the bunch. A 17.5 would be really slow in comparison.

The guys talking up the 17.5 for stock are using it with LiPo (7.4V) in pan cars. Basically, they're saying LiPo+17.5 is about the same speed as 4cell+27T brushed. So, really, unless you're doing pan car stuff, forget about the 17.5 as stock.

In 12th scale, the gap between a 13.5 and 27T stock seems to be a lot more pronounced. I don't know why, maybe the lighter cars favor the BL motors? It's not clear, but considering 13.5 "stock" may only really apply to sedans and electric off-road.