The stainless would probably be easier to keep clean, but the tile might look nicer, depending on your personal preference. Right now we have a piece of sheet metal behind our rangetop, but it's temporary. I have 4x8 slate pieces to put up permanently. I will likely miss the ease of cleaning the sheet metal, and considered keeping it (replacing it with stainless), but even though I'm terminally practical, I just can't get myself to like the colder look. The stone is an important part of the overall look and feel I want for the room. Your subways would be easier to clean than my uneven slate.

Have you looked at various kitchens, seeing it done both ways, to see which you prefer? Check out the Finished Kitchens Blog slideshow and/or the backsplash slideshow linked on the Finished Kitchens Blog page...I'm sure you'll see both options there.

Here is a link that might be useful: Finished Kitchens Blog

Tile would look nicer (if that's the look you want of course--stainless looks perfectly nice as well but you let your preference show :) ).

Stainless is easier to clean but not by a lot--you have to get stainless REALLY clean because every spot shows. Tile, on the other hand, cleans well enough (modern grouts are great) and if you miss a day or a smudge it won't be nearly as obvious.

So, I would personally prefer to be responsible for keeping TILE looking nice than stainless. Because even if it isn't, it's hard to tell. :)

Here's a couple of pics from my kitchen, showing what a subway tiled backsplash looks like behind the range:

{{!gwi}}

For me, the tile backsplash is super easy to clean (and it gets splashed with dinner more frequently than I'd like to admit). On the other hand, I find my stainless fridge hard to keep clean. So, I'm glad to have the tile backsplash.

We have a quilted stainless steel backsplash because I had heard horror stories about keeping the grout clean. Posters here don't seem to have that problem. I love the stainless though - the look and the ease of cleaning. Ours is a little softer look due to the quilting. It is not at all difficult to keep clean.
Here's a look at ours.

{{!gwi}}