Both are gud. But replacing ( in case of breakage) tiles is easier and cheaper than marble slabs.
My suggestion is Marble, as it needs less maintenance, you can always re polish , it lasts for 10-15 years at least. Comparing to this, tiles cannot be polished more than once. That too if its super charged vitrified tiles.
Obviously marble be the first choice. It needs less maintenance than tiles. But in case of breakage tiles is cheaper than marble. Therefore, the decision is yours but looking to use tiles then go for vitrified tiles.