The decision to use ceramic tile in lieu of linoleum or vice-versa is a function of aesthetics, cost and maintenance.
In as much as aesthetics are concerned, ceramic tile offers a virtually infinite number of ways to enhance your already tiled shower. Although some people may argue that tile leaves a bathroom floor feeling cold, there are few finishes that offer the same luxurious impression that tile does. And, considering that your floor consists of a concrete slab, chances are that tile would be no colder than linoleum.
Although you could tile directly over the existing linoleum, we believe that this practice is unacceptable in an area exposed to water.
Under no circumstances should plywood or underlayment be applied over existing linoleum, and then tile installed on top of that. This can result in a less-than-solid substrate, and can cause cracking somewhere down the line. It may also leave you with a height problem at the doorway and at cabinets or plumbing fixtures. We suggest that you install the floor tile directly over the concrete slab.
You will be relieved to know that the cost for a ceramic tile floor installed in a thin-set adhesive will be only slightly higher than a good grade of inlaid vinyl. This would not be true, however, if you had a wood subfloor. If you did, we would strongly suggest the tile be installed in a mortar bed. The mortar bed not only offers an added layer of protection between the tile and the wood floor, but "stiffens" the surface to significantly reduce deflection and the possibility of cracked tile.
It's true that a ceramic tile floor will generally require more ongoing maintenance than a conventional vinyl floor, but you will have a difficult time finding a surface as durable as tile is.