Are you planning to do the tiles yourself or pay someone else to put them in? If you compare the cost of DIY tiles vs installed slab, it's way, way cheaper to do the tiles. For example, in my kitchen if I have someone install a slab it'll cost ~$4000, but I could easily do it for $500-$1000 if I do tiles myself depending on which tiles I choose, obviously that cost would go up a lot if someone else does it. To save the most money though don't get the tiles from the big box stores, you can find better prices at tile or marble/granite stores. The thing to keep in mind is the tiles are much thinner than the granite slab, so they're going to be easier to damage later, plus you have the grout lines which some people don't like. If you decide to go with a slab, the way to save money is to see if you can find leftover pieces at the granite store--you'll end up with some seams in your countertop but you'll save money over having a whole custom-cut slab (you're basically buying the leftovers after someone else had a custom cut slab). The look will be a lot the same but you save a bunch. And don't get a fancy edge treatment--that can add a lot to the cost.
We just had slab granite installed last month. It is beautiful. We are not handy enough to do our own tile, but I have a neighbor who did it and it looks really nice.
The tiles are far more limited in patterns and they tend to be quite plain. If you like a lot of "movement" in the granite you will need a slab. I didn't think I wanted a lot of variation until I went to a slab yard. Wow! What fun! I was hooked. I needed a granite slab and I needed it bad!!! I love the variation. We have been finding all sorts of designs in it. There is a tug boat, a drawing on a cave wall, a papoose, the "eye" on Lord of the Rings. It is fund to see so many different things.