On which weather conditions should I choose my bicycle with studded/spike tyres?
I've got the same tyres in 700C, but only run one bike (in this city). I put them on a couple of weeks ago and they'll come off when the weather warms up in spring. There's no harm in using them all winter. In fact on a thick layer of wet leaves they're better than my marathon plus. They are noisy, but you get used to it and it doesn't down out the traffic. 

One of the worst times round here for sheet ice is frosty Monday mornings, which often follow a sunny Sunday afternoon when people wash their cars. The residential road that doesn't drain very well is one of the few that isn't gritted on my route. 

You're in a slightly different position to me of course. I suggest a simple rule: if it was forecast to be below about 3C at any point overnight, use winter tyres. If there's visible frost on the ground, use winter tyres. If you're planning on coming back late and the forecast is for less than about 3C, use winter tyres.