Boleros only work under certain conditions, i.e. for me,< 55 or so in the AM, and only with sleeveless jerseys. I find my bolero (a Terry, well made and it fits me) just doesn't do well with short sleeved jerseys, only sleeveless. The ss jerseys kind of bunch up around the shoulders and just look funny with it. So, the only time I really have worn my bolero consistently is when we were in Spain and it was 50 or so in the AM and 80-95 in the afternoon. Mine is the original version, with a thermal inside, but they do have a lighter weight one.
I would vote for the arm warmers. More versatile. If you pair them with a vest and/or sleeveless base layer, they will work for a wider range of temperatures, and they can be peeled off when it gets warm. If you can find ones that are women specific and come in sizes, they work fine. Otherwise, they do fall down.