I use Schwalbe Marathon Plus tyres and recently snapped two or three cheap plastic tyre levers trying to get to the gooey centre of my tyre (to fix a puncture). Fortunately I was at home and could go back to get my Park Tools blue plastic levers. I cycled to work. When I was ready to leave, I found my tyre flat again; I must have done a poor job so I set about fixing it and one of my Park Tools levers broke! Luckily I was at work so I could use the communal repair kit which contained something like these:
http://www.wiggle.co.uk/park-tools-tl5c-heavy-duty-steel-tyre-levers/
They're a little big and heavy but they felt like they could lever the tyres off a tractor!
A cheaper variant is here but I don't know if they're a compromise on quality:
http://www.evanscycles.com/products/fwe/pro-steel-tyre-lever-set-ec053194
I've also found nylon-covered metal levers:
It isn't clear whether the Draper ones have a steel core extending to the tip (seems pointless if they don't though). It would be pretty expensive to try them all out so I'm trying to figure out my best bet. My bike is presumably made out of carbon steel so weight isn't an issue and I'm not bothered about scratching my (steel) rims but I'm very concerned about damaging my inner tubes!