Use sanded grout as the "backer" for the wider areas meaning a first shallow coat and then finish with unsanded grout on top. You get the best of both worlds, but it is a lot more labor intensive and I would expect them to charge additional for both the additional materials and labor. Grout crumbles when there is too much water in the mix. It can also affect the color, making it too light or too blotchy.

Our experience was exactly as GreenDesigns describes. Thank the heavens your job entails only a backsplash. Ours was three entire tub walls. The differences in our grout lines were because the tile edges aren't straight lines; rather wavy and not straight angles. The areas he used spacers are even more varied. Wider joints require sanded (the unsanded would crack quickly in wide joints).

Too much water, eh? The grout I mixed myself was very stiff, and I used the exact amount of water called for. I mixed up an entire 7-lb. box just to grout five tiles. I'd never mixed grout in my life and used my fingers to apply it to my sample board. However, nearly four days later, it's really really hard. I'm sincerely hoping the grouts on the contractor's boards were mixed in small quantities, in haste, solely to provide me with a color choice the following day. In any event, I have both my sample and theirs, mine rock solid, and theirs crumbling to sand. I will say, this, not that.

Linelle, there is premixed grout which really saves time and guess work and error free. Costs a little more but is well worth it IMO. Good luck!

When we mixed the grout (unsanded)for our backsplash, it was very stiff, making it a pain (more work) to press into the 1/16" joints with the float. We really had to work to get it applied. I don't know if it's possible, but maybe they mixed in a little more water to make it easier to apply on the sample boards. When we made our sample boards, we mixed up small amounts of the grout by hand to use, as we were just wanted to check the color. We mixed the grout used on the actual installation with the drill and an attachment for mixing concrete type materials. You might show them the sample boards and ask why it's crumbly.

Thanks everyone for your replies. A more pleasing color than the last one I chose and a grout that's lasting are all I'm asking for. :)

Anna, yes, the sample boards will be on display and I'll for sure ask why theirs is crumbly and mine (a total rookie) is nice and solid.

My understanding is that 1/8" is the break point; most directions call for unsanded for 1/8" and sanded for larger than 1/8". Given that you also have small joints, I would use unsanded (or else use GreenDesign's suggestion).

When I want to mix up a small batch, I actually weigh out the grout or thinset using a postal or kitchen scale. The ratio of water to cement is important. (I suspect you're correct about their small batches.) And properly mixed grout is pretty stiff.

Unsanded grout in place. My call. So far, beauteous.