The iPhone 6 is a good few months away. If you can live with the lumia for 6 months or so, then wait for it. If you miss iOS and the apps, and you cant go that long without them, get the 5S.
Though, what are your upgrade options? are you on the 2 year contract upgrade cycle?
And are there any rumored features about the iPhone 6 that you want to have? (i.e. larger screen). You dont want to get the 5S and be kicking yourself and have to wait another 2 years until the iPhone 6s or 7 comes out. But seeing as youve had an iphone 3GS for so long, I dont think you'd care about not having the latest and greatest. I think the 5S will suit you.