>>33496661Thanks for the advice. I actually forgot about the special attack boosting items, so maybe using one of them can make up for the special attack points I'd lose by choosing Timid instead of Modest.
By the way, since posting my question, I found a site that lets you input a Pokemon and a nature and see what Pokemon will outspeed it. Here it is, in case anyone else has been wondering about that sort of thing.
http://www.pokemon-tools.com/xy/en/calc_speed/I used it to see what will outspeed a Modest Jolteon, and it does look like a lot more Pokemon would outspeed it compared to a Timid one. I think with both that data and your advice in mind, Timid might be best after all, because it greatly reduces the number of Pokemon that can outspeed Jolteon.