The only reason for choosing one over the other should be aesthetics. Both will clean up just as easily.

Traction. The tile on the shower floor should ideally provide some traction when wet, for safety, unless you plan to use a shower mat of some type. I actually took a spray bottle of water to the tile shop to test 'slipperiness when wet' properties of the tiles I was considering for my shower. Hand's down, the mattes provided better traction (less slippery) when wet.

I think that the matte finish will be less likely to show water spots than the shiny tile.

We went with matte on the floor and shiny on the walls. Originally we didn't want shiny, but after putting a sample in the space we realized having the shiny walls reflect the light made the shower feel brighter.

the matte won't show water spots as easily but it never has that sparkly clean look... shiny/polished tile looks SO nice when it IS clean i think... (same difference as satin finishes and polished on faucets...) wiping down the shiny tile with a towel easily keeps it 'sparkly'.

Thanks for the responses!

My next question should be how long did it take you to find the "right" tile! There are so many choices!

I see from the responses that it is really a personal choice, no right or wrong answer with the exception of making sure there is enough traction on the shower floor. So its back to the tile shop for the 3rd time....oy!

We're also trying to find the right tile for our shower, since we're currently redoing the master bath and will later need to tile the shower in the guest room. For some reason tile is the one thing I can't seem to decide!

We may split the difference and go with shiny in the guest room, which is more traditional/white, and go with a matte natural stone in the master, since it's more of a rustic spa feel. Maybe you can have it both ways and do the same? :)

Good luck!

Ha! We did shiny on the shower floor and matte on the walls! But our shiny tile is 2X2 mosaic so the grout lines provide traction. I got a sample sheet and got it wet, then assesed slipperiness. Bottom line is that with the grout lines it didn't matter so much.
Now the bathroom floor is done in the matte tile of the shower walls to allow more traction when wet.