We occasionally get mold here in our bathrooms -- I really have to keep on top of the cleaning and make sure to run the bathroom fan to help keep the mold from becoming a problem. That said, I think that tile grout is no harder to keep clean.
On my white grout, I use Soft Scrub with Bleach and a handheld scrub brush -- I just take these in and clean when I'm in the shower once a week. On the bathroom DH is remodeling, we'll have tan grout and so I'll probably use Tilex to clean it.
The most important thing is to use the right grout, seal it well 2-3 times and keep your bathroom clean (if you start noticing mildew or mold, get in and clean thoroughly right away).
Also, be careful to get tile that does not have little pock marks in the surface -- I had tile like that on the floor of my last kitchen and my current kitchen came with a similar tile (lucky me). No matter how much regular cleaning you do, dirt and grime settle into the little pock marks and the only thing I found that really got the tile clean was a thorough scrubbing on my hands and knees. Ugh!
The tile we are installing in the kids' bathroom right now is a nice porcelain tile (good wear rating). We are putting it on the floor and tub surround -- roughly 100 sq ft in all. The bigger tile is 6" x 6" and DH is making a decorative strip about 2/3 of the way up the shower with angled 6" x 6" tiles and matching 2" x 2" tiles. The 6" x 6" tiles were about $6/sqft and the 2" x 2" tiles were about $13.50/sqft. It's costing us about $1000 for the tub surround and floor, including grout, sealer, tiles, etc.
I have to agree that you might want to look at the solid surfaces available. In our last house, we had solid surfaces in the shower -- it's plastic that's drawn out in sheets -- so big plastic walls, but they look like marble and clean like a dream! I used a mild abrasive (often just baking soda) and a scrub brush to clean them once a week and they looked great! In Utah, the company that made them, installed them -- took a few hours out of one day for them to install the tub surround in the basement we had finished.