I had exactly the sam dilema at the start of the year. Kona Explosif frame, with a choice of 16" or 18". I ordered the 16" initially and it looked small. So samll in fact it measured out only slightly larger than my fiancee's 14.5" Orange. The 18" as it turns out was slighty smaller than my 17.5" Gary Fisher, so I got this. These size differences it turns out was because the seat tube extends above the join with the top tube and the measurement taken is for the whole length of the tube, not that that's relevant here.
What you really need to do is try them out and see what feels most comfortable. Also think about what style of riding you are going to do. The smaller frame will be much nicer to ride on twisty technical stuff, but if its too small you might find it uncomfortable on long rides. If it turns out you really are bang in between the two sizes don't be afraid to start checking out other bikes. Chances are there'll be one out there thats perfect for you.