They wouldn't make a plastic one if it didn't work! Metal ones rot like ****, that's why they went to plastic :wink:
The metal protectors have been made of galvanised steel. I would not worry about them rusting, because (1.) it is fairly simple to replace them, and (2.) by the time these plates start to rust, I would rather worry about rust on the painted body parts of the SLK.
The reason for using a plastic shield as the default factory setup cannot have ben the avoidance of rust; if that were the case then the R170 should have had a fiber glass body or a better rust protection in general. More likely, the choice for a plastic engine protection shield was driven by the desire to save both costs and weight (helping to improve the fuel economy somewhat and the weight/road tax classifications for some countries).
For most roads the plastic shield will probably do fine, but since the TS asked for opinions, I shared mine.
It has been many years since I have driven in Turkey (where the TS resides) and at the time the roads would definitely have justified a metal engine cover. I took that into account when giving my opinion.
On our Dutch roads, potholes in and large stones on the roads are rare, but we have plenty of nasty speed bumps. When I purchased my R171, I asked the dealership to install a metal protective cover underneath my R171's engine (replacing the default plastic one). My risk aversion was driven by the fact that cracking the engine means totalling the car. If the TS is willing to take the risk, then I would say: Go ahead. Again, however, I would opt for other measures to improve the fuel economy.
The total weight of the R170 SLK200 is about 1,300kgs; for me the limited percentage of weight reduction (you take out about 13kgs, equalling 1% of the total weigth) simply does not offset the dramatically increased risk of totalling the car. Would there be a 1:1 relationship between weight en fuel economy, it means you may drop from e.g. 9L/100km to 8.91L/100km. I find that negligible. Always driving around with less than half a tank of fuel has a bigger impact (18 litres of petrol also weigh approx. 13 kgs.). Alternatively, look for tyres that are more fuel economic (if you are willing to accept the increase in noise), or adjust your driving behaviour.