Porcelain tiles are processed in factories and baked from refined clay. The process makes them hard and non-porous and a clear choice for exterior use. Marble tiles are porous as they're natural stone, and are best suited for interior walls and moderate traffic floors.
Porcelain tiles are cheaper and even easier to maintain, but marble tiles are often more sought after in spite of their high maintenance and price as they are natural stone and have a unique design.
Marble tiles are made from natural marble stone. Marble is sawed on both sides, polished and finished on the face-up side for a uniform thickness. Since marble tiles are porous, they need to be sealed before use and resealed frequently. They also stain easily. However, since they are natural stones, they offer a unique natural design with different colors and variations in every tile.
Porcelain tiles are usually only made from white clay and they have color throughout the tile. They are denser and hence more suitable for harsher applications like flooring. They are less likely to absorb moisture and so more resistant to staining. However, refined clay makes them harder, and thus more brittle.
This is a good YouTube video demonstrating the key differences between porcelain and natural stone marble tiles:
Marble tiles can be used on walls, but they are more popular for flooring, washbasins, kitchen islands and cooking counters. While they're mostly used as indoor tiles, marble has been used outside the house but in moderate to low traffic areas shielded from rain and freezing cold weather.
Porcelain tiles are hard and are used for flooring as well as interior and exterior walls, as they can weather the cold and moisture. They are quite brittle and may require an expert to cut them.
Since marble is a natural stone, marble tiles have very unique designs and consequently a very high aesthetic value.
Porcelain tiles are less absorbent, hence more stain-resistant. They're also more durable as they can withstand moisture and harsher conditions.
Marble is porous and tends to absorb more moisture; it also stains very easily. Besides, care and maintenance for marble flooring can be quite tedious and consuming. Because of its porosity, it cannot be used for exterior walls and landscaping.
Porcelain tiles are processed and baked with refined clay which makes them hard and quite brittle.
Marble flooring needs installation by an expert and has to be sealed at installation and resealed from every couple years. Marble floor have to be constantly dusted and mops to keep them from staining and absorbing water.
In comparison, porcelain tiles do not need as much maintenance. They have to be cut by an expert as they're quite hard. In fact, they can weather the cold and moisture, and even thrust and pressure more than marble tiles, making it perfect for high-traffic areas and exterior walls.
Porcelain tiles are typically cheaper than marble tiles. Marble tiles are quite often custom ordered and may get even more expensive.
Current prices for some of the commonly used tiles are available on Amazon.com: